Fig. 66.

It is noted by Box ('Practical Treatise on Heat,' p. 166) that an exit for the moist air should not be placed at the top of a drying-chamber, but at the bottom, since in the first case, the hot dry air tends to rise at once to the opening, and pass away unsaturated with moisture, while that cooled by evaporating water from the goods, being heavier, tends to form downward currents and remain in the chamber. To this it may be objected that aqueous vapour is much lighter than air; this is true, other things being equal, but in practice the evaporation of a given quantity of water cools the air and makes it heavier in a materially greater degree than the admixture of aqueous vapour lightens it. This source of waste of heat exists in the turret drier, but is there, from its great height, reduced to a minimum. In lower sheds it becomes very material, and the air currents formed are productive of much harm by causing irregular drying. This difficulty has been met by Mr. Edward Wilson, of Exeter, to whom the leather trade owes several very useful inventions, by an ingenious drying-room constructed on the lines indicated by Box, though I do not know that he was in any way indebted to that writer for the idea. In this Mr. Wilson arranges the steam-pipes, instead of on the floor, in a vertical compartment partitioned from the chamber, through which air is admitted and heated. This hot air fills the top of the chamber and from its lightness floats in a horizontal layer, only descending and escaping by apertures in the floor as it becomes cooled by evaporating the moisture of the hides. Mr. Wilson states that the method answers well in practice, and it is certainly the most scientific in conception, but it might be feared that, as applied to a single floor, the upper parts of the butts, suspended near the ceiling, would dry more rapidly than those near the floor. If applied to a double-floored building, this disadvantage would, from the stronger draught, and consequent larger supply of air, be less likely to show itself, and the upper floor with its uniform warm air would be well adapted for drying off finished sole-leather, while the cooler and milder drying of the ground floor would be fitted both in character and situation for that wet out of the yard. Special precaution would be needed to prevent the heated air escaping by doors opening into the upper floor. There is little doubt that as regards heat this is the most economical system which has yet been invented.

A method has been introduced in the United States of drying wet and finished leather all together, in drying-rooms heated to a considerable temperature, and closely shut up. This is found to answer fairly on leather from sour liquors, but that from strong and sweet liquors is darkened, as might be expected. The drying is accomplished in much shorter time than by the turret drier. The mixture of wet and dry leather, and the lack of ventilation produce an atmosphere nearly saturated with moisture, and hence the drying is not nearly so harsh as might be supposed from the considerable temperatures made use of. There does not, however, seem anything in the principle to recommend its general adoption.

Another invention, of which we have as yet heard little definite in England, consists in drying at a low temperature by air artificially deprived of its moisture. This may be accomplished in several ways. Experiments have been made in drying in a closed chamber provided with trays of calcium chloride to absorb the moisture evaporated. Air when artificially cooled by compression and subsequent expansion, as in the case of ice-making machines, parts with a large portion of its moisture, which is condensed in the form of ice in the tubes of the machine. Such air, if subsequently warmed, would dry powerfully and rapidly.

Before leaving the subject of drying-sheds, a few words on the mechanics of drying in general may not be out of place. Air-drying is dependent on the condition that the air must be capable of taking up more moisture than it already contains. It is a matter of common experience that there are warm days when the air is so saturated with moisture in the form of invisible vapour, that scarcely any drying takes place; and similarly, cool dry days, when leather dries rapidly. The relative amount of moisture in the air is easily ascertained by the simple instrument known as the wet and dry bulb hygrometer; an instrument which ought to be in every drying-shed, especially where steam heat is used. It consists of two similar thermometers, side by side, of which one has the bulb covered with muslin and kept wet by a piece of lamp-cotton attached to it, and dipping in a cup or bottle of water. This water evaporates more or less rapidly, according to the dryness of the air; and as heat is consumed by it in passing into the gaseous condition, the wet thermometer falls more or less below the dry in proportion to the rapidity of the evaporation. On a summer's day, the difference may amount to 9°-12° F. (5°-7° C.), and this is about the extreme dryness permissible in a drying-room for finished leather. Wet leather should of course be dried much more slowly. The influence of heat on drying is two-fold. It increases the capacity of the air for moisture, and it replaces the heat consumed by evaporation. The following tables give the capacity of air for moisture at different temperatures, and the percentage of saturation as shown by the wet and dry thermometer. At Greenwich, the mean humidity for the year is 82 per cent.; or for the day-time only 76 per cent., varying from 62 in summer to 86 in winter:—

Table I.—Capacity of Air for Moisture.

Temp. Fahr.Weight in Pounds of
a Cub. Ft. of
Dry Air.		Weight in Pounds of
Moisture contained in
a Cub. Ft. of
Saturated Air.
32°·0807·000304
42·0791·000440
52·0776·000627
62·0761·000881
72·0747·001221
82·0733·001667
92·0720·002250
102·0707·002997

Table II.—Hygrometer Table.

Temperature
of Air.		Degrees between Wet and Dry Thermometers.
123456789101112
32° F.8775....................
42928578726660544944403633
62948882777267625854504744
82959085807672686460575451
Per cent. of moisture, saturation being 100.