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:--
Skin with hair 1·450 " depilated with lime 1·425 " depilated with sodium sulphide 1·441
but does not give details how these figures were obtained, and if corrected for ash and fat.
The density of limed and puered sheepskin, determined by drying the skin over sulphuric acid until the weight was constant, then determining the volume in petroleum, gave the following results:--
By determining the volume of the wet skin in the volumenometer, and the per cent. of water on drying the skin, the calculated densities were--
Correcting for ash and fat, the dry ash-free skins had the densities--
The density of the wet limed skin was 1·063, but the density calculated from the results (I.) above is 1·0475; from this, it is evident the fibres of the swollen limed skin undergo compression on swelling, or that the water contained in them is in a state of compression, in the same way as gelatin swollen with water occupies a less volume than the sum of the volumes of gelatin and water. Lüdeking[50] found for 10 per cent. gelatin jelly δ = 1·069; δ calculated was 1·041. He attributed the whole of the compression to the water, so that 1 c.c. of water in a 10 per cent. gelatin jelly occupied a volume of 0·96069 c.c. See also par. 4 p. 68.
[50] Wiedemann’s Annal d. Physik, xxxv. p. 352 (1888).
It may be of interest to give the figures used in the preceding lines in tabular form.
---------------------------------------+----------+------------ |Limed skin|Puered skin ---------------------------------------+----------+------------ Density of wet skin | 1·063 | 1·053 Per cent. of water found | 80·63 | 78·2 " " calculated | 79·50 | 79·9 " of ash on wet skin | 1·71 | 1·17 " of fat " " | 0·151 | 2·6 Density of dry ash-free skin substance | 1·3970 | 1·3356 ---------------------------------------+----------+------------