VALUATION OF FEEDING STUFFS 227 



amounts of digestible protein and of thermic or dynamic 

 energy according as the food is intended for maintenance or 

 production they can obtain in the various foods on the 

 market for a given sum of money. 



It is almost superfluous to say that this is rarely or never 

 done. For one thing, the sellers will not, as a rule, consent 

 to dispose of the goods on this basis. The prices of home- 

 grown products hay, cereals, etc. vary according to 

 " quality " so far as it can be determined by inspection 

 colour, smell, hardness. This, however, is often more a 

 matter of vague sentiment or local prejudice than of nutritive 

 value. In the case of the more highly concentrated and more 

 expensive foods cakes, meals, etc. the " quality " is generally 

 decided mainly by the composition, i.e. the total percentages 

 of crude protein, fat, and soluble carbohydrates. 



To say that the prices of the several nutrients vary in 

 different foods is only to repeat that some foods are cheaper 

 than others for equal amounts of the corresponding ingredients ; 

 moreover, the prices fluctuate from time to time and place 

 to place. The relative values (prices), however, it may be 

 assumed, remain constant, and are the same for all foods. 

 They can only be very roughly evaluated, but it is found that, 

 on the average, the relative prices per pound of crude protein 

 and crude fat are about the same ; that of the soluble carbo- 

 hydrates is relatively less, about two-fifths of the price of the 

 others. This may be expressed in the form of an equation 

 as follows : 



j = (2-5P-j-2- 5 F-f C) 



where s is the price, in shillings, per ton, P, F, and C are the 

 percentages of crude protein, fat, and soluble carbohydrates 

 respectively, and n is a number which is different for each 

 food, and which varies according to the state of the market. 



The sum of the terms within the bracket ^^P-^^F-j-C) 

 is called the number of " food-units." Since s = n (food- 

 units), therefore n = ^. , . . In a word, n is the price 



per unit. It roughly expresses the relative pecuniary value 

 of the different foods. The following example illustrates the 



