346 



ON THE PLACE OF FISH IN 



in a com 

 pound is 

 known by 

 separating 

 it out as 

 ammonia. 



The weight The weight of N in a compound is ascertained by 

 in a^com- ent separating it out in the form of ammonia which is a 

 compound of N with H, the only one they form to- 

 gether. This is accomplished by heating some of it 

 with caustic soda, which causes all the N to pass off 

 in ammonia. 



The simple fact that ammonia is given off in such a 

 process can easily be known from smell by heating 

 any substance which contains much N, a piece of 

 cheese for example, with caustic soda, which can be 

 obtained at any druggist's. 



To secure that all the ammonia given off is col- 

 lected without waste, special apparatus is of course 

 required. 



This is not intended as a book of instruction for 

 doing chemical operations, but it is hoped that the few 

 homely experiments described on pp. 341 to 346 

 will suffice to enable those who take the trouble to do 

 them, to realise that C, H, O, and N are actual weigh- 

 able forms of matter, and to understand the nature of 

 the work, sketched in the next few pages, only in out- 

 line, by which facts have been learned about foods 

 and the uses of different kinds of them. 



Of all the immense numbers of elements and 

 compounds, our knowledge of which is frequently 

 increasing through the industry of experimental 

 chemists, all that it is essential to pay attention to for 

 our immediate purpose is that the union of 



H with O produces the compound water. 



C O carbonic acid. 



N and H' 



ammonia. 



