ELEMENTS OF COMPOSITION. 31 



water; they have therefore been regarded as hydrates of carbon. Al] 

 have at least six atoms of the latter. Should they contain more, they 

 are still complete multiples of six. They may be looked upon as deriva- 

 tives of the six-atomed alcohols, and are divided according to their com- 

 position into three groups. 



I. The grape-sugar group (C 6 H 12 6 ). These may be regarded from the 

 way in which they conduct themselves as aldehydes of the six-atomed 

 alcohols. 



II. The cane-sugar group (C 12 H 22 O n ), may be looked upon as made up 

 of anhydrites or ethers, formed of two molecules of the last, with the loss 

 of one molecule of H 2 0. 



III. Cellulose group (C 6 H 10 5 ). The molecular proportions of these 

 compounds are not yet ascertained. Most of them appear to possess a 

 higher molecular weight than the last. They are likewise anhydritic 

 derivatives. 



All hydrocarbons are of neutral constitution; none are volatile; 

 some are crystalline ; some are insoluble ; others easily soluble in water. 

 The latter appear in the system usually in watery solution, or possibly 

 entering into the composition of other substances as so-called glucosides. 



The various hydrocarbons pass easily one into another, and in these 

 processes many of the albuminous matters of the body play an impor- 

 tant part as ferments. On being digested in dilute mineral acids, most 

 of them are converted into grape sugar. The relation, further, of the 

 hydrocarbons to the organic acids is of importance, many of them 

 possessing the same empyrical composition, and a part springing easily 

 from the latter, as in the case of acetic and other fatty acids, and lactic 

 acid. Their relations to alcohols must also be remembered. 



The importance of the hydrocarbons in the vegetable kingdom is very 

 great, produced as they are, with some exceptions (as, for instance, sugar 

 of milk), by the plant ; some of them are likewise of great histogeriic 

 worth here, especially cellulose. The case is quite otherwise in the ani- 

 mal organism, particularly in the bodies of the higher animals and man. 

 The few hydrocarbons which here appear do not manifest tissue-forming 

 qualities in the remotest degree, and are dissolved in the juices; on the 

 contrary, they seem to be decomposition products of other materials, 

 such as the protein compounds, or their source is the food taken into 

 the body. By their physiological decomposition they yield, according to 

 the commonly received theory, carbonic acid and water. It is still an 

 undecided question how far they can pass into the higher members of 

 the fatty acids, thus playing a part in the formation of fat, but that some 

 of the hydrocarbons do so is certain. 



In this group are included among several other substances three kinds 

 of sugar, namely, grape sugar, inosite, and sugar of milk, as constituents 

 of our body. 



The sugars are generally sweet to the taste, soluble in water, and 

 almost all crystalline. They undergo vinous fermentation with greater 

 or less readiness quickly in the case of grape sugar ; slowly, as in. that 

 of sugar of milk ; or not at all, as in that of inosite. 



Glycogen, C^H 10 5 . 



This compound, standing between amylum and dextrin, was dis- 

 covered by Bernard. It is met with as an amorphous substance, which 



