342 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. 



They can Chemical compounds may pass through many 



nised by* ^' changes, in none of which can the elements be re- 

 thenToutf 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. 

 An example The gas we burn in our houses will furnish a simple 



of separating 



out. 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 



