We will now turn our attention to the chemical composition of wool and fur fibres. On chemical analysis still another element is found over and above those mentioned as the constituents of silk fibre. In silk, you will recollect, we observed the presence of carbon, hydrogen, oxygen, and nitrogen. In wool, fur, etc., we must add a fifth constituent, namely, sulphur. Here is an analysis of pure German wool--Carbon, 49.25 per cent.; hydrogen, 7.57; oxygen, 23.66; nitrogen, 15.86; sulphur, 3.66--total, 100.00. If you heat either wool, fur, or hair to 130 deg. C., it begins to decompose, and to give off ammonia; if still further heated to from 140 deg. to 150 deg. C., vapours containing sulphur are evolved. If some wool be placed in a dry glass tube, and heated strongly so as to cause destructive distillation, products containing much carbonate of ammonium are given off. The ammonia is easily detected by its smell of hartshorn and the blue colour produced on a piece of reddened litmus paper, the latter being a general test to distinguish alkalis, like ammonia, soda, and potash, from acids. No vegetable fibres will, under any circumstances, give off ammonia. It may be asked, "But what does the production of ammonia prove?" I reply, the "backbone," chemically speaking, of ammonia is nitrogen. Ammonia is a compound of nitrogen and hydrogen, and is formulated NH_{3}, and hence to discover ammonia in the products as mentioned is to prove the prior existence of its nitrogen in the wool, fur, and hair fibres. _Action of Acids on Wool, etc._--Dilute solutions of vitriol (sulphuric acid) or hydrochloric acid (muriatic acid, spirits of salt) have little effect on wool, whether warm or cold, except to open out the scales and confer roughness on the fibre. Used in the concentrated state, however, the wool or fur would soon be disintegrated and ruined. But under all circumstances the action is far less than on cotton, which is destroyed at once and completely. Nitric acid acts like sulphuric and hydrochloric acids, but it gives a yellow colour to the fibre. You see this clearly enough in the fur that comes from your furriers after the treatment they subject it to with nitric acid and nitrate of mercury. There is a process known called the stripping of wool, and it consists in destroying the colour of wool and woollen goods already dyed, in order that they may be re-dyed. Listen, however, to the important precautions followed: A nitric acid not stronger than from 3 deg. to 4 deg. Twaddell is used, and care is taken not to prolong the action more than three or four minutes.