The weight 
of N present 
in a com- 
pound is 
known by 
separating 
it out as 
ammonia. 
16 ON THE PLACE OF FISHIN 
The weight of N in a compound is ascertained by 
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. I1 to 16 
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. 
Cine.) ao » -s carbonic acid. 
N and H - fi ammonia. 
