THE CHEMICAL COMPOSITION OF FOODS 19 



be well to indicate very briefly the methods by which the 

 relative proportions of the proximate principles of a food 

 are determined. 



1. Water. — A known weight of the substance is taken 

 and dried in a water oven for several hours at 100° C 

 until it ceases to lose weight. The loss in weight repre- 

 sents the water which has been evaporated off, and its 

 proportion of the whole weight of substance originally 

 taken can be easily stated as a percentage. The residue 

 is the dry substance. 



2. Nitrogenous Matter. — This is estimated in various 

 ways, of which the following method is one. A known 

 weight of the dried substance is mixed with strong sul- 

 phuric acid, placed in a flask, and heated. By this 

 means the whole, or practically the whole, of the 

 nitrogen is driven off in the form of ammonia (Kjeldahl's 

 method). Now, proteins contain from 15 to 18 per cent, 

 of nitrogen, or on an average 16 per cent., so that, by 

 multiplying the amount of nitrogen by -yw-^ or 6'25, the 

 amount of protein is arrived at. Since the protein 

 nitrogen content varies, this method would not be 

 accurate under any circumstances, and it is, further, 

 unsound in that it takes no count of the existence of 

 amides. However, it sufficiently indicates the procedure 

 followed. To separate proteins from amides it is neces- 

 sary to precipitate the proteins by means of metallic 

 salts, such as mercuric chloride, or copper sulphate, or 

 by tannic acid. The amides are not affected by these 

 substances, and remain in solution. 



3. Fats. — The finely powdered and dried substance is 

 subjected to the solvent action of either petroleum spirit 

 or ether, which dissolves out the fats and oils, together 

 with small amounts of waxes, resins, and colouring 



