206 ANIMAL AMIDES. 



the mixtures are exposed to the solar rays, a certain portion of 

 the metals is reduced. Sulphate of platinum precipitates collin 

 in brown viscid flocks, which become black in drying, and may 

 then be easily reduced to powder. Mr Edmond Davy, to whom 

 we owe the knowledge of this precipitate, informs us that it is 

 composed of 



Peroxide of platinum, 56-11 or 14 = 1 atom. 



Sulphuric acid, . 20-02 or 5 1 atom. 



Collin and water, . 23-87 



100-00 



When the solution of tannin is dropped into collin, a copious 

 white precipitate appears, which soon forms an elastic adhesive 

 mass, not unlike vegetable gluten. This precipitate is composed 

 of gelatin and tannin ; it soon dries in the open air, and forms a 

 brittle resinous-like substance, insoluble in water, capable of re- 

 sisting the greater number of chemical agents, and not suscepti- 

 ble of putrefaction. It resembles exactly overtanned leather. The 

 precipitate is soluble in the solution of gelatin, as Davy first ob- 

 served. Neither is the whole tan thrown down, unless the solu- 

 tions both of tannin and gelatin be somewhat concentrated. 

 Tremulous gelatin, as was first observed by the same chemist, 

 does not precipitate tannin ; but if we employ a solution of gela- 

 tin so strong that it gelatinizes when cold, and heat it till it be- 

 comes quite liquid, it answers best of all for throwing down tan- 

 nin. It is by this property of forming a white precipitate with 

 tannin that gelatin is usually detected in animal fluids. It is not, 

 however, a perfectly decisive test, as albumen is also thrown down 

 by tannin. But collin is precipitated by tannin when in a much 

 more dilute state than albumen. A solution of one part of col- 

 lin in 5000 parts of water is sensibly precipitated by tannin. 

 When we mix a hot concentrated liquid solution of collin with 

 infusion of nutgalls, a white, curdy precipitate falls, which, if 

 there be an excess of tannin, forms a coherent elastic mass, which 

 constitutes a horizontal layer on the bottom of the vessel. It is 

 insoluble in water and alcohol ; though both of these liquids de- 

 prive it of a little tannin. When dry, it is black, hard, brilliant, 

 and breaks with a vitreous or rather resinous fracture. In water, 

 it softens and assumes its original appearance. According to 

 Davy, it is composed of 



