A HARD-WORKING DIET. 341 



somewhat complicated. But it is really not difficult, 

 it requires only a little careful attention, principally in 

 guarding against a confusion of ideas from attaching to 

 terms meanings not intended ; and it is far easier to 

 follow when practical acquaintance has been made with 

 C, H, O, and N by even simple experiments with them. 



It may help to clear away a preliminary difficulty to C H O N 



cannot be 



mention that although, as just stated, every one has recognised as 

 been living on C, H, O, and N, all their lives, they tne C om- 

 could not see the individual elements, not even the {^ Q s u * at 

 solid carbon, in the compounds as they occur in food - 

 foods. These can only be got at by separating them 

 out by chemical analysis at different temperatures. 



It is just the same as with many familiar things we just as 

 do not use as food. In brass, for example, which is an 

 alloy of the elements copper and zinc, the copper and 

 zinc cannot be recognised as such, though they can be msed ln brass * 

 separated out ; or in bronze the tin and copper cannot . 

 be recognised, though they can be separated out. 

 Elements cannot be recognised in a chemical 

 compound, though they may be in a mechanical 

 mixture. A simple experiment that can be made on The meaning 

 a shovel over a fire will serve to illustrate what is ^^ < 2 

 the difference that is meant by the terms " mechanical com P und - 

 mixture" and "chemical compound." Get some 

 fine copper filings and some powdered sulphur well 

 mixed together on a sheet of paper. The copper 

 and the sulphur can still be separately distinguished 

 if not with the unaided eye, they can be with a 

 magnifying glass and the sulphur can be washed 

 away. This is called a " mechanical mixture." But 

 put such a mixture on a shovel over the fire till it 

 glows, and on cooling there will be found a black 



