The author has examined the action of dilute H2SO4 on sheep skins, and this may be considered typical of the action of acids on skin generally.
When puered and drenched sheep skin is treated for 20 minutes with a very dilute solution of H2SO4 ( 1/280), it swells up considerably and becomes soft and semi-transparent, The fibres of the skin, which under normal conditions have a diameter of 4µ–6µ become 20µ or more; on staining with picro-carmin there are seen to be certain fibres which are unaffected or only slightly affected by the acid. These are the elastic fibres[119] and the capillary blood vessels.
On filtering a solution of the above strength in which skin in excess has been treated for 20 minutes, and evaporating 100 c.c. to dryness in a platinum dish, a residue of 0·4992 grm. was obtained. A second experiment gave 0·496 grm. of which 61 per cent. was organic matter.
By Kjeldahling another portion of the residue, the N in 0·12 grm. amounted to 0·0122 grm. equivalent to 0·073 grm. of skin substance; i.e. 60 per cent. of the soluble matter is skin. It is evident that the matter in solution is a compound of skin substance and H2SO4 together with a little soluble mineral matter contained in the skin. The amount of substance dissolved depends upon the condition and previous treatment of the skin.
In the presence of NaCl, which prevents the swelling of the fibres, puered sheep grains take up 7·0 c.c. of (N/1) H2SO4 from a solution containing 17 c.c. of (N/1) H2SO4 (0·833 grm. H2SO4) and 7·8 grm. of NaCl per 100 c.c. About 300 grm. wet skin per litre were used. The skin substance dissolved by the acid in the time named does not appear to be more than that dissolved by the weak acids of a drench, although an equivalent weight of H2SO4 has a much more powerful swelling action on skin than these acids.
A piece of puered and drenched sheep skin was carefully washed in distilled water and then dried in vacuo until it ceased to lose weight; the absolute dry weight was 4·2670 grm. The skin was soaked down in distilled water and again drenched for 30 hours in a clean bran drench, developing approximately 0·8 grm. lactic acid, and 0·5 grm. acetic acid per litre (see this Journal, May 1893). It was again washed and dried in vacuo. It weighed 4·12 grm., a loss of 0·14 grm. = 3·44 per cent.
The action, therefore, of the acids is a reducing one, inasmuch as skin is dissolved, but as the fibres hold a certain amount of acid, the skin appears to be plumped. Where the acid is a mineral one the skin tans plump, but with a brittle and inelastic fibre, weak organic acids make a plump, soft leather with a moderately elastic fibre, giving to the skin a somewhat india-rubber feel. We thus see that weak acids alone do not produce a similar result to a dung bate, which causes the skin to fall and produces a flat soft leather with a stretch in it, which will not spring back.
I have also tried the action of the following Na and NH3 salts upon skin:—
Sodium lactate, C3H5O3Na, prepared by neutralising 2 grm. lactic acid with sodium carbonate in 1000 c.c. water. The solution was used under the same conditions as the other bating experiments, viz. at 37° C. the reducing action was nil, but on prolonging the digestion, the medium was found to be very suitable for the development of putrefactive organisms, and, as a consequence, the skin was attacked and partly peptonised.