CHEMICAL EXPERIMENTS.
As the permanent gases head the list, they will first engage our attention, beginning with the element oxygen—Symbol O, combining proportion 8. There is nothing can give a better idea of the enormous quantity of oxygen present in the animal, vegetable, and mineral kingdoms, than the statement that it represents one-third of the weight of the whole crust of the globe. Silica, or flint, contains about half its weight of oxygen; lime contains forty per cent.; alumina about thirty-three per cent. In these substances the element oxygen remains inactive and powerless, chained by the strong fetters of chemical affinity to the silicium of the flint, the calcium of the lime, and the aluminum of the alumina. If these substances are heated by themselves they will not yield up the large quantity of oxygen they contain.
Nature, however, is prodigal in her creation, and hence we have but to pursue our search diligently to find a substance or mineral containing an abundance of oxygen, and part of which it will relinquish by what used to be called by the "old alchemists" the torture of heat. Such a mineral is the black oxide of manganese, or more correctly the binoxide of manganese, which consists of one combining proportion of the metal manganese—viz., 27.6, and two of oxygen—viz., 8 × 2 = 16. If three proportions of the binoxide of manganese are heated to redness in an iron retort, they yield one proportion (equal to 8) of oxygen, and all that has just been explained by so many words is comprehended in the symbols and figures below:—
3 MnO2 = Mn3O4 + O.
Thus the 3 MnO2 represent the three proportions of the binoxide of manganese before heat is applied, whilst the sign =, the sign of equation (equal to), is intended to show that the elements or compounds placed before it produce those which follow it; hence the sequel Mn3O4 + O shows that another compound of the metal and oxygen is produced, whilst the + O indicates the liberated oxygen gas. The iron retort employed to hold the mineral should be made of cast iron in preference to wrought iron, as the latter is very soon worn out by contact with oxygen at a red heat. A gun-barrel will answer the purpose for an experiment on the small scale, to which must be adapted a cock and piece of pewter tubing. Such a make-shift arrangement may do very well when nothing better offers; but as a question of expense, it is probably cheaper in the end to order of Messrs. Simpson and Maule, or of Messrs. Griffin, or of Messrs. Bolton, a cast-iron bottle, or cast-iron retort, as it is termed, of a size sufficient to prepare two gallons of oxygen from the binoxide of manganese, which, with four feet of iron conducting-pipe, and connected to the bottle with a screw, does not cost more than six shillings—an enormous dip, perhaps, in the juvenile pocket, and therefore we shall indicate presently a still cheaper apparatus for the same purpose. (Fig. 93.)
Fig. 93.
a. The iron bottle, containing the black oxide of manganese, with pipe passing to the pneumatic trough, b b, in which is fixed a shelf, c, perforated with a hole, under which the end of the pipe is adjusted, and the gas passes into the gas-jar, d.
The oxygen is conveyed to a square tin box provided with a shelf at one end, perforated with several holes at least one inch in diameter, called the pneumatic trough; any wooden trough, butter or wash-tub, foot-pan or bath, provided with a shelf, may be raised by the same title to the dignity of a piece of chemical apparatus. The gas jar must be filled with water by withdrawing the stopper and pressing it down into the trough, and when the neck is below the level of the water, the stopper is again inserted, and the jar with the water therein contained lifted steadily on to the shelf, the entry of atmospheric air being prevented by keeping the lower part of the gas jar, called the welt, under the water. Sometimes the pneumatic trough contains so small a quantity of water that on raising the gas jar to the shelf the liquid does not cover the bottom, and the air rushes up in large bubbles. Under these circumstances it is better to provide a gallon stone jug full of water, so that when the jar is being raised to the shelf it may be thrust into the trough (on the same principle as the crow and the pitcher in the fable), and thus by its bulk (as the stones in the pitcher) raise the water to the proper level. When the gas jar is about half filled with gas the jug may be withdrawn. This arrangement saves the trouble of constantly adding and baling out water from the pneumatic trough. (Fig. 94.)
Fig. 94.
a a. Pneumatic trough, with gas jar raised to shelf; bubbles of air are rushing in at b, as the level of the water is below the shelf—viz., at c c. d d. Same trough and gas jar with water kept over the shelf by the introduction of the stone pitcher e, full of water.
There are other solid oxygenized bodies in which the affinities are less powerful, and hence a lower degree of heat suffices to liberate the oxygen gas, and one of the most useful in this respect is the salt termed chlorate of potash. If the substance is heated by itself, the temperature required to expel the oxygen is almost as high as that demanded for the black oxide of manganese; but, strange to say, if the two substances are reduced to powder, and mixed in equal quantities by weight, then a very moderate increase of heat is sufficient to cause the chlorate of potash to give up its oxygen, whilst the oxide of manganese undergoes no change whatever. It seems to fulfil only a mechanical office—possibly that of separating each particle of chlorate of potash from the other, so that the heat attacks the substance in detail, just as a solid square of infantry might repel almost any attack, whilst the same body dispersed over a large space might be of little use; so with the chlorate of potash, which undergoes rapid decomposition when mixed with and divided amongst the particles of the oxide of manganese; less so with the red oxide of iron, and still less with sand or brick-dust. (Fig. 95.)
Fig. 95.
Preparation of oxygen from chlorate of potash and oxide of manganese.
KO.ClO5 = KCl + O6.
This curious fact is explained usually by reference to what is called catalytic action, or decomposition by contact (κατα, downwards, and λυω, I unloosen), being a power possessed by a body of resolving another into a new compound without undergoing any change itself. To make this term still clearer, we may notice another example in linen rags, which may be exposed for any length of time to the action of water without fear of conversion into sugar; if, however, oil of vitriol is first added to the linen rags, and they are subsequently digested at a proper temperature with water, then the rags are converted into sugar (the author has seen a specimen made of an "old shirt"); but, curious to relate, the oil of vitriol is unchanged in the process, and if the process be commenced with a pound of acid, the same quantity is discoverable at the end of the chemical decomposition of the linen rags, and their conversion into sugar.
If a mixture of equal parts of oxide of manganese and chlorate of potash is placed in a clean Florence flask, with a cork, and pewter, or glass tube attached, great quantities of oxygen are quickly liberated, on the application of the heat of a spirit lamp. Such a retort would cost about fourpence, and if the flask is broken in the operation it can be easily replaced by another, value one penny, as the same cork and tube will generally suit a number of these cheap glass vessels. Corks may always be softened by using either a proper cork squeezer, or by placing them under a piece of board or a flat surface, and rolling and pressing the cork till quite elastic.
Whilst fitting the latter into the neck of a flask, it is perhaps safer to hold the thin and fragile vessel in a cloth, so that if the flask breaks the chemical experiment may not be arrested for many days by the severe cutting and wounding of the fingers. After the cork is fitted, it is to be removed from the flask and bored with a cork borer. This useful tool is sold in complete sets to suit all sizes of glass tubes, and the pewter or glass being inserted, the flask and tube will be ready for use, provided the tube is bent to the proper curve. This is easy enough to perform with the pewter, but not quite so easy with the glass tube, which must be held over the flame of a spirit lamp till soft, and then bent very gradually to the proper curve. If a short length of the glass tube is heated, it bends too sharply, and the convexity of the glass is flattened, whilst the internal diameter of the tube is lessened, so that at least three inches in length should be warmed, and the heat must not be continued in one place only, but should be maintained in the direction of the bend, the whole manipulation being conducted without any hurry. (Fig. 96.)
Fig. 96.
a. The cork squeezer. b. The cork borers. c. The operation of bending the glass tube over the flame of the spirit-lamp. d. The neck of the flask, with cork and tube bent and fitted complete for use.
Having filled a gas jar with oxygen, it may be removed from the pneumatic trough by sliding it into a plate under the surface of the water, and to prevent the stopper being thrust out accidentally from the jar by the upward pressure of the gas, whilst a little compressed, during the act of passing it into the plate, it is advisable to hold the stopper of the jar firmly but gently, so that it cannot slip out of its place. A number of jars of oxygen may be prepared and arranged in plates, all of which of course must contain a little water, and enough to cover the welt of the jar.