The usual methods of determining density are well known,[49] and consist in weighing the body first in air, then in water or other liquid. If m be the weight of the body, and it loses the weight w when weighed in water—
δ = m/w.
For experiments on skin, instead of weighing in water it has been found more convenient to use a simple volumenometer, which was devised by Mr. Douglas J. Law (see Coll. 1911, p. 230.)
Fig. 8.—Volumenometer for Raw Skin.
The apparatus (Fig. 8) consists of the two vessels A and B, connected by means of thick rubber tubing to the burette C, of which the top is enlarged to a bulb. The bottle A, which was specially made for us by Messrs. Townson and Mercer, London, is of about 1 litre capacity, and the wide mouth is closed by a stopper, accurately ground to fit the neck, and extending down to the bottom of the neck. The upper part of the stopper is elongated to a tube, which is closed by the tap G. The vessel A is also fitted with a tap E. The vessel B serves as a reservoir, and is used to adjust the level of the liquid in the burette C by means of the tap at F. To find the volume of a piece of skin, the method of procedure is as follows. The bottle A is filled with water up to the neck, and the stopper D, carefully greased, is inserted. The tap E is then closed, and the burette is filled with water. Then, with the taps G and E open, the bottle A is filled with water up to D by raising the burette. G and E are then closed, and by opening the tap F the level of the water in the burette is adjusted to zero when F is again closed. G and E are then opened again, and, by lowering the burette, the water in A is allowed to fall below the level of the neck of the bottle. E is then shut, the stopper D removed, and the piece of skin is carefully introduced, avoiding air bubbles. The stopper D is then replaced with the tap open, and, by opening E and raising the burette, the water is allowed to come up to the stopper D again. The taps G and E are then closed, and the volume of the piece of skin read off directly from the burette scale. Volumes up to 50 c.c. are measurable as described above, but larger volumes may be measured if a known volume of water is run from the burette into the reservoir before introducing the skin.
By using various solutions in the apparatus instead of pure water, the real swelling or contracting effect of these upon the skin may be observed. The skin is introduced into the bottle A, and the solution adjusted to G, which is then closed; then, by leaving the tap E open, the real swelling or contraction of the skin is measured by the rise or fall of the liquid in the burette.
Petroleum or other liquid may be used instead of water; in some cases, the use of petroleum is more advantageous.
The density of dry gelatin as determined by Lüdeking is 1·412, which is not greatly different from that of skin. Carini gives the following figures for ox-hide:—