CU/ton College Scientific Society. 59 



respective colours, red and yellow prussiate of potasli. In order 

 to obtain tliein we take, according to Dr Lankester on the ' Uses 

 of Animals in Relation to the Industry of Man,' p. 155, ' three 

 sets of substances: — 1. Refuse of animal substances — blood, bones, 

 hoofs, horns, &c., which yield nitrogen and carbon. 2. Old scraps 

 of iron — refuse iron, shoes from dead horses, rusty nails, and 

 worn-out iron hoofs. 3. Potashes, Montreal ashes, the refuse, if 

 you like, of hewn trees ; and these supply the potassium. Now, 

 when these things are exposed to heat together, they arrange them- 

 selves in this way : the iron unites with the carbon and nitrogen 

 in the form of cyanogen, forming ferro-cyanogen ; and this com- 

 pound unites with the potassium, forming the ferro-cyanide of 

 potassium ; and this is what these salts are — ferro-cyanides.' 



Passing over the intestines and the tendons, of which the former 

 are used to cover sausages, while the latter are boiled down to 

 form jelly, glue, &c., we will go on to the hide. 



This is to us a most important part of the horse, since it is the 

 source, in common with that of other animals, of leather. What 

 a strait we should be in if our supply of leather should be ex- 

 hausted ! Of course we might to some extent find a substitute 

 for it, and indeed such a substitute is extensively used in the form 

 of gutta-percha and india-rubber; but neither these, nor any other 

 material with which we are acquainted, answers so perfectly as 

 leather for many purposes. Great efforts have been made to apply 

 india-rubber to the manufacture of mill-bands, for conveying 

 motion from one wheel to another, but it does not equal leather. 

 Then, again, our shoes and the harness for our horses are made of 

 leather, and it would be a difficult matter to procure an equally 

 efficient material for these purposes. A great part of the body of 

 carriages, trunks and portmanteaus either partly or altogether, 

 the hose for our fire-engines, the backs of books, the covers of 

 chairs, picture-frames, and a hundred other necessaries or comforts 

 of life, depend on the leather trade. What, then, is leather ? Well, 

 chemically speaking, it would be said to consist of a tannate of 

 gelatine and albumen, the former predominating. Some say, 

 however, that it is not gelatine but ossein which forms so large a 

 part of the skin, since a skin is not soluble in water as gelatine is. 



Probably most people have been at one time or other in a 

 country where oak-trees are abundant, and they have very likely 

 seen numbers of these trees, after they are cut down, stripped of 

 their bark, which is piled up in heaps close to them. This bark 

 is the principal source of the tannic acid which is used to convert 

 the hide or skin into leather. The process by which the change 

 is effected is called ' tanning.' The ordinary method of tanning 

 is to put the hides into a pit with alternate layers of oak-bark, 



