The fibres of the skin have only a limited capacity for holding acids, and soon begin to swell abnormally and partially dissolve. Reimer[118] has found that the material may be reprecipitated by lime water. It forms a fibrous mass, which has not the sticky feel of gelatin, but is at once converted into that body by boiling. [118] H. K. Procter, “Text Book of Tanning,” 1885, p. 18. The author has examined the action of dilute H_{2}SO_{4} 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 H_{2}SO_{4} (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. [119] See Procter, “Text Book of Tanning,” pp. 8 and 21. 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 H_{2}SO_{4} 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) H_{2}SO_{4} from a solution containing 17 c.c. of (N/1) H_{2}SO_{4} (0·833 grm. H_{2}SO_{4}) 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 H_{2}SO_{4} 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.