They can 
only be recog- 
nised by 
separating 
them out. 
An example 
of separating 
out. 
12 ON. THE PLACE OF FISH IN 
substance, differing in properties from both copper 
and sulphur. This is an example of what is called a 
“chemical compound.” Neither the copper nor the 
sulphur can now be distinguished, but they are there 
and can be again separated out by proper chemical 
means. 
Chemical compounds may pass through many 
changes, in none of which can the elements be re- 
cognised, and yet it can be proved they are there by 
their being separated out afterwards. To take one 
example only, the well-known “blue vitriol,’ or 
“copperas” of the oil shops. It is a compound of 
copper and sulphur, but neither of them can be dis- 
tinguished whether the compound is in the state of 
the yellow solution, or of the greenish blue crystals, 
or of the white powder to which these crystals turn 
on heating. Yet the copper and the sulphur can be 
obtained in their original state and original quantity 
by chemical separation. Many illustrations of this 
kind might be given, but any one of them rightly 
understood will make it easier to comprehend the 
nature of the more obscure changes our food com- 
pounds pass through both in being prepared as foods 
by plants and animals, and also within our bodies 
after we have taken them as foods. The C,H, O and 
N can always be “separated out” at any stage, though 
they pass through many complicated combinations. 
We “live” only so long as C, H, O and N are under- 
going combinations within us. 
The gas we burn in our houses will furnish a simple 
illustration of “separating out” or “splitting up.” 
It is, leaving out impurities, Carbon and Hydrogen 
in a gaseous state. When a gas tap is turned on 
