63 HYDROCARBONACEOUS PROXIMATE PRINCIPLES. 



two different conditions ; starch being the substance in the form of a 

 solid deposit, and glucose in the form of solution and activity. 1 In 

 the animal body, the glycogen of the liver is converted into soluble 

 glucose, and thus enters the circulation before it takes an active part 

 in the nutritive operations; and vegetable starch, when taken as food, 

 undergoes the same transformation in the intestinal canal. Finally 

 these substances, from whatever source they may be derived, are com- 

 pletely decomposed in the interior of the system, and do not reappear, 

 in any notable quantity, in the excreted fluids of the body. 



IV. Fats. 



The fatty matters, or fixed oils, are distinguished from the preceding 

 group, so far as regards their chemical composition, by the fact that they 

 do not contain hydrogen and oxygen in the proportions to form water, 

 the oxygen being present in smaller quantity ; and also by their large 

 proportion of carbon, which preponderates much, by weight, over the 

 other two elements. This fact is probably connected with the strongly 

 marked inflammability which constitutes one of their most useful pro- 

 perties, the oils being decomposed at a temperature of 300 (570 F.), 

 and burning with a bright flame. The peculiarly smooth consistency 

 of the oleaginous matters is also one of their distinguishing features, 

 and enables them to be employed as lubricating substances, to diminish 

 the friction between opposite surfaces. 



The fats are all insoluble in water, slightly soluble in alcohol, and 

 freely soluble in ether, which is accordingly used with advantage in 

 extracting them from their admixture with other organic matters. 

 They are also readily soluble in each other. They exhibit no rotatory 

 action upon polarized light. They are all fluid at a high temperature, 

 and crystallize on being cooled down to the requisite point ; the precise 

 degree at which crystallization takes place varying for the different 

 kinds of fats. 



The fats are not only insoluble in water, but they refuse to mix with 

 it, even after prolonged mechanical agitation ; and as soon as the two 

 fluids are left at rest they separate from each other, the water remain- 

 ing below, and the oil rising to the surface, where it collects as a dis- 

 tinct layer. But if the watery fluid contain a trace of free alkali, the 

 oil is broken up into minute particles, which are disseminated uni- 

 formly throughout the fluid and held in permanent suspension. Such 

 a fluid is called an emulsion, and presents an opaque white color, owing 

 to the intimate mixture of watery and oleaginous particles having 

 different refractive powers. In an emulsion, the oil does not suffer any 

 chemical modification, but is simply broken up into a state of minute 

 dissemination. It can be recovered, with all its original characters, 

 by evaporating the watery fluid and extracting the oil from the dry 

 residue by means of ether. Oil may also be emulsioned by contact 



1 Sachs, Trait do Botauique. Puris, 1874, p. 840. 



