ALIMENTATION. 79 



tlie oils and tlie carbohydrates. Both, consist of carbon, 

 hydrogen, and oxygen, but the oils have a much greater 

 number of equivalents in their chemical formulae than the 

 carbohydrates; that is to say, they consist of more complex 

 arrangements of atoms ; they have likewise a larger amount 

 of carbon and a smaller proportion of oxygen in their compo- 

 sition. The carbohydrates are so called from consisting of 

 carbon in conjunction with hydrogen and oxygen, in the 

 proportion in which they are combined in water; they 

 include starch, together with the allied substance cellulose, 

 which forms a large part of growing vegetables, and sugar. 

 Starch is a highly important constituent of vegetable diet, 

 forming the larger part of the weight of flour, and a much 

 greater proportion of the substance of potatoes. It occurs 

 in small granules which are insoluble in cold water, but 

 which burst when boiled, the contents of the starch granule 

 being dissolved, while the outer envelope remains unacted 

 on. It is a property of starch that in the presence of certain 

 substances, sometimes termed amylolytic ferments, it becomes 

 converted into grape sugar ; and no starch is capable of being 

 absorbed into the system until it has undergone that change. 

 The sugars used in food are of three principal descriptions 

 cane sugar, grape sugar, and sugar of milk. It may be here 

 remarked that milk, the sole food provided by nature for the 

 infant, consists of a mixture of a solution of proteid sub- 

 stance in the form of casein, oil in the form of butter, sugar 

 of milk, and various salts, and therefore contains all the 

 varieties of aliment necessary for health. 



51. Properly to nourish the body, the daily waste must be 

 balanced by daily repair, and the food must contain a suffi- 

 ciency of every substance required by the tissues. It follows 

 from this, and has been proved by experiment, that no 

 amount of carbonaceous food will make up for want of nitro- 

 genous ingredients. On this account, animals which feed on 

 vegetables have to consume large quantities of food that they 

 may extract the requisite amount of nitrogenous substance; 

 and persons who feed entirely on potatoes, require to use 

 much greater quantities than they would require of other 

 diet, because potatoes consist principally of starch, and have 

 remarkably little nitrogenous substance in their composition. 



