DRYING ORGANIC BODIES.
12. Volatile Bodies.—In agricultural analysis it becomes necessary to determine the percentage of bodies present in any given sample which is volatile at any fixed temperature. The temperature reached by boiling water is the one which is usually selected. It is true that this temperature varies with the altitude and within somewhat narrow limits at the same altitude, due to variations in barometric pressure. As the air pressure to which any given body is subjected, however, is a factor in the determination of its volatile contents, it will be seen that within the altitudes at which chemical laboratories are found, the variations in volatile content will not be important. This arises from the fact that while water boils at a lower temperature, as the height above the sea level increases, the corresponding diminished air pressure permits a more ready escape of volatile matter. As a consequence, a body dried to constant weight at sea level, where the temperature of boiling water is 100°, will show the same percentage of volatile matter as if dried at an altitude where water boils at 99°. When, therefore, it is desirable to determine the volatile matter in a sample approximately at 100°, it is better to direct that it be done in a space surrounded by steam at the natural pressure rather than at exactly 100°, a temperature somewhat difficult to constantly maintain. However, where it is directed or desired to dry to constant weight exactly at 100°, it can be accomplished by means of an air-bath or by a water-jacketed-bath under pressure, or to which enough solid matter is added to raise the boiling-point to 100°. It is not often, however, that it is worth while to make any special efforts to secure a temperature of 100°. When bodies are to be dried at temperatures above 100°, such as 105°, 110°, and so on, an air-bath is the most convenient means of securing the desired end. The different kinds of apparatus to be employed will be described in succeeding paragraphs.
13. Drying at the Temperature of Boiling Water.—The best apparatus for this process is so constructed as to have an interior space entirely surrounded with boiling water or steam, with the exception of the door by which entrance is gained thereto. The metal parts of the apparatus are constructed of copper, and to keep a constant level of water and avoid the danger of evaporating all the liquid, it is advisable to have a reflux condenser attached to the apparatus. It is also well to secure entrance to the interior drying oven, not only by the door, but also by small circular openings, which serve both to hold a thermometer and to permit of the aspiration of a slow stream of dry air through the apparatus during the progress of desiccation. The gaseous bodies formed by the volatilization of the water and other matters are thus carried out of the drying box and the process thereby accelerated. The bath should be heated by a burner so arranged as to distribute the flame as evenly as possible over the base. A single lamp, while it will boil the water in the center, will not keep it at the boiling-point on the sides. The temperature of the interior of the bath will not therefore reach 100°. The interior of the oven should be coated with a non-detachable carbon paint to promote the radiation of the heat from its walls, as well as to protect the parts from oxidation where acid fumes are produced during desiccation. Instead of a reflux condenser a constant water level may be maintained in the bath by means of a mariotte bottle or other similar device.
Figure 5. Water-jacketed Drying Oven.
When a bath of this kind is arranged for use with a partial vacuum, it should be made cylindrical in shape, with conical ends, as shown in [fig. 5], in order to bear well the pressure to which it is subjected. Among the many forms of steam-baths offered, the analyst will have but little difficulty in selecting one suited to his work. To avoid radiation the exterior of the apparatus should be covered with a non-conducting material.
Figure 6. Thermostat for Steam-Bath.
14. Drying In a Closed Water Oven.—When it is desired to keep the temperature of a drying oven exactly at 100° instead of at the heat of boiling water, a closed water oven with a thermostat is to be employed. The oven should be so constructed as to secure a free circulation of the water about the inner space. Since as a rule the water between the walls of the apparatus will be subjected to a slight pressure, these walls should be made strong, or the cylindrical form of apparatus should be used. The thermostat used by the Halle Station is shown in [Fig. 6].[4] A ⋃ shaped tube, with a bulb on one arm and a lateral smaller tube sealed on the other, is partly filled with mercury and connected by rubber tubes on the right with the gas supply, and on the left with the burner. The end carrying the bulb is connected directly by a rubber and metal tube with the water space of the oven. This device is provided with a valve which is left open until the temperature of the drying space reaches about 95°. The tube conducting the gas is held in the long arm of the ⋃ by means of a cork through which it passes air-tight and yet is loose enough to permit of its being moved. Its lower end is provided with a long ▲ shaped slit. When the valve leading to the water space is closed and the water reaches the boiling point, the pressure of the vapor depresses the mercury in the bulb arm of the ⋃ and raises it in the other. As the mercury rises it closes the wider opening of the ▲ shaped slit, thus diminishing the flow of gas to the burner. By moving the gas entry tube up or down a position is easily found in which the temperature of the drying space, as shown by the thermometer, is kept accurately and constantly at 100°.
In a bath arranged in this way a steam condenser is not necessary. Since, however, in laboratories which are not at a higher altitude than 1,000 feet the boiling-point of water is nearly 100°, it does not seem necessary to go to so much trouble to secure the exact temperature named. There could be no practical difference in the percentage of moisture determined at 100°, and at the boiling-point of water at a temperature not more than 1° lower.
15. Drying in an Air-Bath.—In drying a substance in a medium of hot air surrounded by steam, as has been described, the process is, in reality, one of drying in air. The apparatus usually meant by the term air-bath, however, has its drying space heated directly by a lamp, or indirectly by a stratum of hot air occupying the place of steam in the oven already described. The simplest form of the apparatus is a metal box, usually copper, heated from below by a lamp. In the jacketed forms the currents of hot air produced directly or indirectly by the lamp are conducted around the inner drying oven, thus securing a more even temperature. The bodies to be dried are held on perforated metal or asbestos shelves in appropriate dishes, and the temperature to which they are subjected is determined by a thermometer, the bulb of which is brought as near as possible to the contents of the dish. One advantage of the air-bath is in being able to secure almost any desired temperature from that of the room to one of 150° or even higher. Its chief disadvantage lies in the difficulty of securing and maintaining an even temperature throughout all parts of the apparatus. Radiation from the sides of the drying oven should be prevented by a covering of asbestos or other non-combustible and non-conducting substance. The burner employed should be a broad one and give as even a distribution of the heat as possible over the bottom of the apparatus.
Figure 7. Spencer’s Drying Oven.
16. Spencer’s Air-Drying Oven.—In order to secure an even distribution of the heat in the desiccating space of the oven, Spencer has devised an apparatus, [shown in the figure], in which the temperature is maintained evenly throughout the apparatus by means of a fan.[5] The oven has a double bottom, the space between the two bottoms being filled with air. The sides are also double, the space between being filled with plaster. The fan is driven by a toy engine connected with the compressed air service or other convenient method. Thermometers placed in different parts of the apparatus, while in use, show a rigidly even heat at all points so long as the fan is kept in motion. The actual temperature desired can be controlled by a gas regulator. This form of apparatus is well suited to drying a large number of samples at once. Portions of liquids and viscous masses may also be dried by enclosing them in bulbs and connecting with a vacuum.
Spencer’s oven can also be used to advantage in drying viscous liquids in a partial vacuum. For this purpose the flask A, [Fig. 7], containing the substance is made with a round bottom to resist the atmospheric pressure. Its capacity is conveniently from 150 to 200 cubic centimeters. It is closed with a rubber stopper carrying a trap, H Hʹ, to keep the evaporated water from falling back. The details of the construction of the trap H are shown at the right of the [figure]. The vapors enter at the lateral orifice, just above the bulb, while the condensed water falls back into the bulb instead of into the flask A. A series of flasks can be used at once connected through the stopcocks G with the circular tube E leading to the vacuum. A water pump easily exhausts the apparatus, maintaining a vacuum of about twenty-seven inches. The hot air in the oven is kept in motion by the fan B, thus ensuring an even temperature in every part. The flask A may be partly filled with sand or pumice stone before the addition of the samples to be dried, and the weight of water lost is determined by weighing A before and after desiccation. If it be desired to introduce a slow current of dry air or some inert gas into A, it is easily accomplished by passing a small tube, connected with the dry air or gas supply, through the rubber stopper and extending it into the flask as far as possible without coming into contact with the contents.
17. Drying Under Diminished Air Pressure.—The temperature at which any given body loses its volatile products is conditioned largely by the pressure to which it is subjected. At an air pressure of 760 millimeters of mercury, water boils at 100° but it is volatilized at all temperatures. As the pressure diminishes the temperature at which a body loses water at a given rate falls. This is a fact of importance to be considered in drying many agricultural products. This is especially true of those containing oils and sugars, nearly the whole number. Invert sugar especially is apt to suffer profound changes at a temperature of 100°, the levulose it contains undergoing partial decomposition. Oils are prone to oxidation and partial decomposition at high temperatures in the presence of oxygen.
In drying in a partial vacuum therefore a double advantage is secured, that of a lower temperature of desiccation and in presence of less oxygen. It is not necessary to have a complete vacuum. There are few organic products which cannot be completely deprived of their volatile matters at a temperature of from 70° to 80° in a partial vacuum in which the air pressure has been diminished to about one-quarter of its normal force.
Figure 8. Electric Vacuum Drying Oven.
18. Electric Drying-Bath.—The heat of an electric current can be conveniently used for drying in a partial vacuum by means of the simple device illustrated in [Fig. 8]. In ordering a heater of this kind the voltage of the current should be stated. The current in use in this laboratory has a voltage of about 120, and is installed on the three wire principle. It is well to use a rheostat with the heater in order to control the temperature within the bell jar. The ground rim of the bell jar rests on a rubber disk placed on a thick ground glass or a metal plate, making an air-tight connection. A disk of asbestos serves to separate the heater from the dish containing the sample, in order to avoid too high a temperature.
19. Steam Coil Apparatus.—For drying at the temperature of superheated steam, it is convenient to use an apparatus furnished with layers or coils of steam pipes. The drying may be accomplished either in the air or in a vacuum. In this laboratory a large drying oven, having three shelves of brass steam-tubes and sides of non-conducting material, is employed with great advantage. The series of heating pipes is so arranged as to be used one at a time or collectively. Each series is furnished with a separate steam valve, and is provided with a trap to control the escape of the condensed vapors. In the bottom of the apparatus are apertures through which air can enter, which after passing through the interior of the oven escapes through a ventilator at the top. With a pressure of forty pounds of steam to the square inch and a free circulation of air, the temperature on the first shelf of the apparatus is about 98°; on the second from 103° to 104°, and on the third about 100°. The vessels containing the bodies to be dried are not placed directly on the brass steam pipes, but the latter are first covered with thick perforated paper or asbestos. For drying large numbers of samples, or large quantities of one sample, such an apparatus is almost indispensable to an agricultural laboratory.
Figure 9. Steam Coil Drying Oven.
A smaller apparatus is shown in [Fig. 9]. The heating part G is made of a small brass tube arranged near the bottom in a horizontal coil and continued about the sides in a perpendicular coil. Bodies placed on the horizontal shelf are thus entirely surrounded by the heating surfaces except at the top.[6] The steam pipe S is connected with the supply by the usual method, and the escape of the condensation is controlled either by a valve or trap in the usual way. The whole apparatus is covered by a bell jar B, resting on a heavy cast-iron plate P, through which also the ends of the brass coil pass. The upper surface of the iron plate may be planed, or a planed groove may be cut into it, to secure the edge of the bell jar. When the air is to be exhausted from the apparatus, a rubber washer should be placed under the rim of the bell jar. The latter piece of apparatus may either be closed, as shown in the [figure], by a rubber stopper, or it is better, though not shown, to have a stopper with three holes. One tube passes just through the stopper and is connected with the vacuum; the second passes to the bottom of the apparatus and serves to introduce a slow stream of dry air or of an inert gas during the desiccation. The third hole is for a thermometer. When no movement of the residual gas in the apparatus is secured, a dish containing strong sulfuric acid S’ is placed on the iron plate and under the horizontal coil, as is shown in the [figure]. The sulfuric acid so placed does not reach the boiling-point of water, and serves to absorb the aqueous vapors from the residual air in the bell jar. By controlling the steam supply the desiccation of a sample can be secured in the apparatus at any desired temperature within the limit of the temperature of steam at the pressure used. Where no steam service is at hand a strong glass flask may be used as a boiler, in which case the trap end of the coil must be left open. The vacuum may be supplied by an air or bunsen pump. When a vacuum is not used an atmosphere of dry hydrogen may be supplied through H.
Figure 10. Carr’s Vacuum Drying Oven.
Figure 10. (Bis) Vacuum Oven Open.
20. Carr’s Vacuum Oven.—A convenient drying oven has been devised in this laboratory by Carr.[7] It is made of a large tube, preferably of brass. The tube may be from six to nine inches in diameter and from twelve to fifteen inches long. One end is closed air-tight by a brass end-piece attached by a screw, or brazed. The other end is detachable and is made air-tight by ground surfaces and a soft washer. In the [figure] this movable end-piece is shown attached by screw-nuts, but experience has shown that these are not necessary. On the upper longitudinal surfaces are apertures for the insertion of a vacuum gauge and for attachment to a vacuum apparatus.
In the [figure] the thermometer and aperture for introducing dry air or an inert gas are shown in the movable end disk, but they would be more conveniently placed in the fixed end. The oven is heated below by a gas burner, which conveniently should be as long as the oven. The heat is not allowed to strike the brass cylinder directly, but the latter is protected by a piece of asbestos paper.
The temperature inside of the oven can be easily kept practically constant by means of a gas regulator, not shown in the figure, or by a little attention to the lamp. For a vacuum of twenty inches a temperature of about 80° should be maintained. When the vacuum is more complete a lower temperature can be employed. This apparatus is simple in construction, strong, cheap, and highly satisfactory in use.
21. Drying in Hydrogen.—In some of the processes of agricultural analysis it becomes important to dry the sample in hydrogen or other inert gas. This may be accomplished by introducing the dry gas desired into some form of the apparatus already described. The drying may either be accomplished in an atmosphere of hydrogen practically at rest or in a more limited quantity of the gas in motion. The latter method is to be preferred by reason of its greater rapidity. The analyst has at his command many forms of apparatus designed for the purpose mentioned above. It will be sufficient here to describe only two, devised particularly for agricultural purposes.
The first one of these, designed by the author, was intended especially for drying the samples of fodders for analysis according to the methods of the Association of Agricultural Chemists.[8]
Figure 11. Apparatus for Drying in a
Current of Hydrogen.
For the purpose of drying materials contained in flasks and tubes in a current of hydrogen the apparatus shown in [Fig. 11] is used. This apparatus consists of a circular box, B, conveniently made of galvanized iron, having a movable cover, S, fitted for the introduction of steam into the interior of the apparatus. Condensed steam escapes at W. A stream of perfectly pure and dry hydrogen enters at H, passes up through the material to be dried, down through the bulb V, containing sulfuric acid, and follows the direction of the arrows through the rest of the apparatus. The stream of hydrogen is thus completely dried by passing through bulbs containing sulfuric acid, on the way from one piece of the apparatus to the other. A, represents a flask such as is used, with the extraction apparatus described. The apparatus which we have used will hold eight tubes or flasks at a time, and thus a single stream of hydrogen is made to do duty eight times in drying eight separate samples. The great advantage of the apparatus is in the fact that the stream of hydrogen must pass over and through the substance to be dried. In order to prevent any sulfuric acid from being carried forward into the next tube the bulb K, above the sulfuric acid, may be filled with solid pieces of soda or potash.
This apparatus has been in use for a long time and no accidents from sulfuric acid being carried forward have occurred, and there is no danger, provided the stream of hydrogen is kept running at a slow rate. If, however, by any accident the stream of hydrogen should be admitted with great rapidity, particles of the sulfuric acid might be carried forward and spoil the next sample. To avoid any such accident as this the proposal to introduce the potash bulb has been made. The apparatus works with perfect satisfaction, and it is believed that when properly adjusted check weighings can be made by weighing the bulbs, showing their increase in weight, which will give the volatile matter, and weighing the flasks or tubes, which will show the loss of weight. The only chance for error in weighing the bulbs is that some of the volatile matter may be material which is not dissolved in sulfuric acid, and is thus carried on and out of the apparatus. The blackening of the sulfuric acid in the bulbs, in the drying of all forms of organic matter, shows that the loss in weight of such bodies is not due to water alone, but also to organic volatile substances, which are capable of being decomposed by the sulfuric acid, thus blackening it.
22. Caldwell’s Hydrogen Drying-Bath.—An excellent device for drying in hydrogen has been described by Caldwell.[9] A vessel of copper or other suitable material serves to hold the tubes containing the samples to be dried. It should be about twenty-four centimeters long, fifteen high, and eight wide. This vessel is contained in another made of the same material and of the dimensions shown in the [figure]. On one side the edge of this containing vessel may not be more than one centimeter high and the bath should rest against it. The other side is made higher to form a support for the drying tubes as indicated.
Figure 12. Caldwell’s Hydrogen Drying Apparatus.
The tube containing the substance a d is made of glass and may be closed by the ground stoppers c b or the tube stoppers e f. At a it carries a perforated platinum disk for holding the filtering felt. The tube should be about thirteen centimeters long and have an internal diameter of about twenty millimeters. With its stoppers it should weigh only a little over thirty grams. The asbestos felt should not be thick enough to prevent the free passage of gas. Passing diagonally through the bath are metal tubes, preferably made of copper, and of such a size as just to receive the glass drying tubes. If these be a little loose they should be made tight by wrapping them with a narrow coil of paper at either end of the tubular receptacle. The entrance of cold air between the glass tube and its metal holder is thus prevented, and the glass tube is held firmly in position. The glass tube should be weighed with its two solid stoppers. Afterwards the sample, about two grams, is placed on the asbestos felt and the stoppers replaced and the whole reweighed. The exact weight of the sample is thus obtained. The solid stoppers are then removed and the tube stoppers inserted. The lower end of the tube is then connected with the supply of dry hydrogen. The upper tube stopper is connected by a rubber tube with a small bottle containing sulfuric acid through which the escaping hydrogen is made to bubble. A double purpose is thus secured; moisture is kept from entering the drying tube and the rate at which the hydrogen is passing is easily noted. After the drying is completed the solid stoppers are again inserted, the tube cooled in a desiccator and weighed. The loss of weight is entered as water. The tube containing the sample can afterwards be put into an extractor and treated with ether or petroleum in the manner hereafter described. This apparatus requires more hydrogen than the one previously described, but it is rather simple in construction, is easily controlled, and has given satisfactory results.
Figure 13. Liebig’s Ente.
23. Drying in Liebig’s Tubes.—In drying samples, especially of fodders, the method practiced at the Halle Station is to place them in drying tubes, the form of which is shown in [Fig. 13]. A stream of illuminating gas, previously dried by passing over sulfuric acid and calcium chlorid, is directed through the tubes.[10] Many of these tubes can be used at once, arranged as shown in [Fig. 14]. When the air is all driven out the stream of gas can be ignited so as to regulate the flow properly by the size of the flame. The tubes are held in drying ovens, as shown in the [figure], the temperature of which should be kept at 105°-107°. The drying should be continued for eight or ten hours. At the end of this time the gas in the tube is to be expelled by a stream of dry air and the tubes cooled in a desiccator and weighed. There are few advantages in this method not possessed by the processes already described. The samples, moreover, are not left in a condition for further examination, either by incineration or extraction.
Figure 14. Drying Apparatus used at the Halle Station.
24. Wrampelmayer’s Drying Oven.—The apparatus used at the Wageningen Station, in Holland, for drying agricultural samples, was devised by Wrampelmayer and is shown in [Fig. 15]. The oven is so constructed as to permit of drying in a stream of inert gas. Illuminating gas is let into the drying space of the oven through the tube A B. At B the entering gas is heated by the same lamp which boils the liquid in the water space of the apparatus. The hot gas is dried in the calcium chlorid tube c and then passes into the oven at D. At E it leaves the apparatus and is thence conducted to the lamp F, used for heating the bath. The lamp should be closed by a wire gauze diaphragm to prevent any possible explosion by reason of any admixture with the air in the oven. The condensation of the aqueous vapors is effected by means of the condenser G. In the drying space is a small shelf holder, which, by means of the hook H, can be removed from the apparatus. The drying space is closed from the upper part of the apparatus, which contains no water by the cover J, resting on a support K. This rim is covered with a rubber gasket L, by means of which the cover J can be fastened with a bayonet latch air-tight. This fastening is shown at N. Being closed in this way the part of the cylindrical oven above the cover may be left entirely open. Instead of the rather elaborate method of closing the bath, some simple and equally effective device might be used. The cover J is best made with double metallic walls enclosing an asbestos packing.
Figure 15. Wrampelmayer’s Oven.
It is evident that this oven could be used with an atmosphere of carbon dioxid or of air, provided the gas for heating were derived from a separate source and the tube between E and F broken. In a drying oven designed by the author, the movable top is made with double walls and the space between is joined to the steam chamber by means of a flexible metallic tube, thus entirely surrounding the drying space with steam.
25. The Ulsch Drying Oven.—A convenient drying oven is described by Ulsch which varies from the ordinary form of a water-jacketed drying apparatus in having a series of drying tubes inserted in the water-steam space.
Figure 16. Ulsch Drying Oven.
The arrangement of the oven is shown in the accompanying [figure]. The water space is filled only to about one-third of its height. When the heat is applied the cock c is left open until the steam has driven out all the air. It is then closed and the temperature of the bath is then regulated by the manometer e, connected with the bath by d. The bottom of the manometer cylinder contains enough mercury to always keep sealed the end of the manometer tube. The rest of the space is filled with water. At the top the manometer tube is expanded into a small bulb which serves as a gas regulator, as shown in the [figure]. The gas is admitted also by a small hole above the mercury in the bulb, so that when the end of the gas inlet tube is sealed enough gas still passes through to keep the lamp burning. With a mercury pressure of thirty centimeters the temperature of the bath will be about 105°. The walls of the bath should be made strong enough to bear the pressure corresponding to this degree. The drying can be accomplished either in the cubical drying box a or in the drying tubes made of thin copper and disposed as shown in the [figure]. The natural draft is shown by the arrows. The substance is held in boats placed in the tube as indicated. The air in traversing the tube is brought almost to the temperature of the water-steam space in which the tube lies. The natural current of hot air can easily be replaced by a stream of dry illuminating or other inert gas.
26. Drying Viscous Liquids.—In the case of cane juices, milk, and similar substances, the paper coil method may be used.[11] The manipulation is conducted as follows: A strip of filtering paper from five to eight centimeters wide and forty centimeters in length, is rolled into a loose coil and dried at the temperature of boiling water for two hours, placed in a dry glass-stoppered weighing tube, cooled in a desiccator and weighed. The stoppered weighing tube prevents the absorption of hygroscopic moisture. About three cubic centimeters of the viscous or semi-viscous liquid are placed in a flat dish covered by a plate of thin glass and weighed. The coil is then placed on end in the dish, and the greater part of the liquid is at once absorbed. The proportions between the coil and the amount of liquid should be such that the coil will not be saturated more than two-thirds of its length. It is then removed and placed dry end down in a steam-bath and dried two hours. The dish, covered by the same plate of glass, is again weighed, the loss in weight representing the quantity of liquid absorbed by the coil. After drying for the time specified the coil is again placed in the hot weighing tube, cooled and its weight ascertained. The increase represents the solid matter in the sample taken. This method has been somewhat modified by Josse, who directs that it be conducted as follows:[12] Filter-paper is cut into strips from one to two centimeters wide and three meters long. The strips are crimped so they will not lie too closely together and then wrapped into coils. These coils can absorb about ten cubic centimeters of liquid. One of them is placed in a flat dish about two centimeters high and seven in diameter, and dried as described, covered, cooled and weighed. There are next placed in the dish and weighed one or two grams of the massecuite, molasses, etc., which are to be dried and the dish again weighed and the total weight of the matter added, determined by deducting the weight of the dish and cover. About eight cubic centimeters of water are added, the material dissolved with gentle warming, the coil placed in the dish, and the whole dried for two hours. The cover is then replaced and the whole cooled in a desiccator and weighed. The increase in weight represents the dry matter in the sample taken.
The above method of solution of a viscous sample in order to divide it evenly for desiccation is based on the principle of the method first proposed by the author and Broadbent for drying honeys and other viscous liquids.[13] In this process the sample of honey, molasses, or other viscous liquid is weighed in a flat dish, dissolved in eighty per cent alcohol, and then a weighed quantity of pure dry sand added, sufficient to fill the dish three-quarters full. The alcoholic solution of the viscous liquid is evenly distributed throughout the mass of sand by capillary attraction, and thus easily and rapidly dried when placed on the bath.
Pumice stone, on account of its great porosity, is also an excellent medium for the distribution of a viscous liquid in aiding the process of desiccation. The method has been worked out in great detail in this laboratory by Carr and Sanborn,[14] and most excellent results obtained. Round aluminum dishes two centimeters high and from eight to ten centimeters in diameter are conveniently used for this process. The pumice stone is dried and broken into fragments the size of a pea before use.
27. General Principles of Drying Samples.—It would be a needless waste of space to go into further details of devices for desiccation. A sufficient number has been given to fully illustrate all the principles involved. In general, it may be said that drying in the open air at a temperature not exceeding that of boiling water can be safely practiced with the majority of samples. For instance, we have found practically no change in this laboratory in the composition of cereals dried in the air and in an inert gas. The desiccation should in all cases be accomplished as speedily as possible. To this end the atmosphere in contact with the sample should be dry and kept in motion. An oven surrounded by boiling water and steam is to be preferred to one heated by air. Constancy of temperature is quite as important as its degree and this steadiness is most easily secured by steam at atmospheric pressure. Where higher temperatures than 100° are desired the steam must be under pressure, or the boiling-point of the water may be raised by adding salt or other soluble matters. A bath of paraffin or calcium chlorid may also be used or a sand or air-bath may be employed. The analyst must not forget, however, that inorganic matters are prone to change at temperatures above 100°, even in an inert atmosphere, and higher temperatures must be used with extreme caution.
Drying in partial vacuum and in a slowly changing atmosphere may be practiced with all bodies and must be employed with some. The simple form of apparatus already described will be found useful for this purpose. At a vacuum of twenty inches or more, even unstable organic agricultural products are in little danger of oxidation. In the introduction of a dry gas, therefore, air will be found as a rule entirely satisfactory. In the smaller form of vacuum apparatus described, however, there is no objection to the employment of hydrogen or of carbon dioxid. The gas entering the apparatus should be dried by passing over calcium chlorid or by bubbling through sulfuric acid. In this laboratory the vacuum is provided by an air-pump connected with a large exhaust cylinder. This cylinder is connected by a system of pipes to all the working desks. The chief objection to this system is the unsteadiness of the pressure. When only a few are using the vacuum apparatus for filtering or other purposes the vacuum will stand at about twenty inches. When no one is using it the vacuum will rise to twenty-eight or twenty-nine inches. At other times, when in general use, it may fall to fifteen inches. Where a constant vacuum is desired for drying, therefore, it is advisable to connect the apparatus with a special aspirator which will give a pressure practically constant.
The dishes containing the sample should be low and flat, exposing as large a surface as possible. For viscous liquids it will be found advisable to previously fill the dishes with pumice stone or other inert absorbent material to increase the surface exposed.
The special methods of drying milk, sirup, honeys, and like bodies, will be described in the paragraphs devoted to these substances.
In drying agricultural products, not only water but all other matters volatile at the temperature employed are expelled. It is only necessary to conduct the products of volatilization through sulfuric acid to demonstrate the fact that organic bodies are given off. In the case mentioned the sulfuric acid will be speedily changed to a brown and even black color by these bodies. It is incontestable, however, that in most cases the essential oils and other volatile matters thus escaping are not large in quantity and could not appreciably affect the percentage composition of the sample. In such cases the whole of the loss on drying is entered in the note book as water. There are evidently many products, however, where a considerable percentage of the volatile products is not water. The percentage of essential oils, which have a lower boiling-point than water, can be determined in a separate sample and this deducted from the total loss on drying will give the water.
Simple as it seems, the determination of water in agricultural products often presents peculiar difficulties and taxes to the utmost the patience and skill of the analyst. Having set forth the substantial principles of the process and indicated its more important methods, there is left for the worker in the laboratory the choice of processes already described, or, in special cases, the device of new ones and adaption of old ones to meet the requirements of necessity.