contains nitrogen.  Thirdly, I will heat some fur with soda-lime.  Ammonia escapes, giving all the reactions described under silk.  Hence fur, wool, etc., contain nitrogen.  As regards proofs of all three of these classes of fibres containing carbon, hydrogen, and oxygen, the char they all leave behind on heating in a closed vessel is the carbon itself present.  For the hydrogen and oxygen, a perfectly dry sample of any of these fabrics is taken, of course in quantity, and heated strongly in a closed vessel furnished with a condensing worm like a still.  You will find all give you water as a condensate—­the vegetable fibre, acid water; the animal fibres, alkaline water from the ammonia.  The presence of water proves both hydrogen and oxygen, since water is a compound of these elements.  If you put a piece of potassium in contact with the water, the latter will at once decompose, the potassium absorbing the oxygen, and setting free the hydrogen as gas, which you could collect and ignite with a match, when you would find it would burn.  That hydrogen was the hydrogen forming part of your cotton, silk, or wool, as the case might be.  We must now attack the question of sulphur.  First, we prepare a little alkaline lead solution (sodium plumbate) by adding caustic soda to a solution of lead acetate or sugar of lead, until the white precipitate first formed is just dissolved.  That is one of our reagents; the other is a solution of a red-coloured salt called nitroprusside of sodium, made by the action of nitric acid on sodium ferrocyanide (yellow prussiate).  The first-named is very sensitive to sulphur, and turns black directly.  To show this, we take a quantity of flowers of sulphur, dissolve in caustic soda, and add to the lead solution.  It turns black at once, because the sulphur unites with the lead to form black sulphide of lead.  The nitroprusside, however, gives a beautiful crimson-purple coloration.  Now on taking a little cotton and heating with the caustic alkaline lead solution, if sulphur were present in that cotton, the fibre would turn black or brown, for the lead would at once absorb such sulphur, and form in the fibre soaked with it, black sulphide of lead.  No such coloration is formed, so cotton does not contain sulphur.  Secondly, we must test silk.  Silk contains nitrogen, like wool, but does it contain sulphur?  The answer furnished by our tests is—­no! since the fibre is not coloured brown or black on heating with the alkaline lead solution.  Thirdly, we try some white Berlin wool, so that we can easily see the change of colour if it takes place.  In the hot lead solution the wool turns black, lead sulphide being formed.  On adding the nitroprusside solution to a fresh portion of wool boiled with caustic soda, to dissolve out the sulphur, a splendid purple coloration is produced.  Fur and hair would, of course, do the same thing.  Lead solutions have been used for dyeing the hair black; not caustic alkaline solutions like this, however.  They would do something more than turn the hair black—­probably give rise to some vigorous exercise of muscular power!  Still it has been found that even the lead solutions employed have, through gradual absorption into the system, whilst dyeing the hair black, also caused colics and contractions of the limbs.