50 



fectly examined, and what we know about it is not 

 the fruit of a direct investigation, but a collection 

 of accidental observations, and from these we have 

 learned, that the cellular texture, when boiled 

 slowly, is dissolved by degrees in great mea- 

 sure, and that the solution, whilst cooling, co- 

 agulates to a jelly and contains glue. Several 

 other animal matters have the same property, as 

 for instance, the cartilage and the skin. These, 

 however, have not the same texture, and pro- 

 bably not the same composition, since they can- 

 not all be completely dissolved with the same 

 ease. This glue does not exist as such in the ani- 

 mal matters, but is a product of the boiling. 

 The erroneous idea, that glue was found within 

 the living body, and was dissolved in its hu- 

 mours, has been supported by the experiment 

 of ascertaining its presence by means of infusion 

 of galls; but although many other animal matters 

 are precipitated by the tanning principle, yet the 

 precipitate with glue has this distinguishing 

 characteristic, that it clogs together into a tough 

 elastic mass, which, when dried, becomes hard and 

 brittle. Such a precipitate cannot be obtained 

 from any animal humours, except from urine, 

 after it has previously been boiled some time 

 with alkali, whereby the dissolved animal mat* 



