The Constituents and Uses of Food. 



them ; while from a function in the body which they alone are 

 competent to perform, they have been designated " flesh- 

 formers." The difficulty of separating the albuminoids in a 

 pure and unaltered state has prevented chemists from unravelling 

 their chemical constitution, which is, however, known to be 

 exceedingly complex. All the albuminoids are either naturally 

 soluble in water, or may be made so by the action of a fixed 

 alkali, the so-called albuminates being thus formed. Their chief 

 element, characterising by its presence the- whole group, is 

 nitrogen — the element which, constitutes seventy-nine parts by 

 measure of common air, which is present in nitre, nitric acid, 

 and ammonia, and which is generally so much more abundant 

 in animal than in vegetable tissues. The percentage of nitrogen 

 in the different albuminoids appears to range between 13 and 

 iS}4, but there are doubts as to the purity of the materials 

 which, when submitted to analysis, have yielded these numbers. 

 The range of variation in the percentage composition of the 

 members of the whole group is shown in this table : 



The amount of albuminoids present in any food is usually 

 ascertained by determining the amount of nitrogen existing in 

 it, and then multiplying the nitrogen found by the factor 6-3. 

 In this procedure two assumptions are made : one, that all 

 albuminoids contain a mean percentage of 15-87 of nitrogen, 

 for 6-3 X i5-87 = 99-98i, practically 100; the other assumption 

 is that no other nitrogen compounds are present. The former 

 assumption may pass, the latter needs a word of explanation. 

 Vegetable and animal foods do contain -nitrogen in non-albu- 

 minoid forms ; for instance, more than half the nitrogen of lettuces, 

 water-cress, and spinach exists in the form of nitrates, which 

 are wholly useless as flesh-formers. Then, too, there are a 



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