NON-NITROGENOUS ORGANIC CELL-CONSTITUENTS. 121 



stained black by perosmic acid ; this reagent is, therefore, a delicate 

 microscopic test for the detection of fats, and, since fat is a constant 

 constituent of nervous tissue, is used as a means of recognizing this 

 tissue. In vegetable cells fat is partly produced directly from C0 2 and 

 H s O, and also through the transformation of starch ; the latter is its 

 mode of origin in oily seeds and fruits where it is stored up until required 

 for germination and growth. 



The natural fats are, without exception, compounds of a triatomic 

 radical, propenyl or glyceryl, combined with three atoms of a monatomic 

 fatty acid, namely, either palmitic, stearic, or oleic acids. The fats 

 formed by the union of these acids with the radical glyceryl are termed 

 palmitin, stearin, and olein. 



A few fats contain other glycerin ethers, such, for example, as are 

 found in butter. At the ordinary temperatures, fats are either solid, like 

 tallow ; semi-solid, like butter and lard ; or fluid, like oils. These differ- 

 ences depend upon the differences in their composition. The more 

 stearin or palmitin there is present, the more the fats tend to solidify ; 

 while the more olein there is, the more fluid are they. All fatty bodies 

 become fluid considerably below the temperature of boiling water. In 

 the pure condition fats are odorless, tasteless, and of alkaline reaction. 

 When kept in contact with the air, they become rancid from the setting 

 free of fatty acids and the oxidation of gljceryl, with the resulting pro- 

 duction of volatile fatty acids and glycerin. In this process they acquire 

 odor and taste, and have an acid reaction. Fats have a lower specific 

 gravity than water. All fats are completely insoluble in water, but when 

 water contains bodies such as gum or albumen in solution, fats will then 

 remain mechanically suspended in the form of an emulsion, which is 

 merely the breaking up of the oil into minute globules. When fluid, fats 

 render paper which is coated with them transparent (grease-spots). 

 Many of the fats are soluble in alcohol, especially when hot, and all are 

 soluble in ether, chloroform, the volatile oils, benzol, and carbon disul- 

 phide. When fats contain small quantities of free fatty acids they will 

 form a permanent emulsion with sodium carbonate solution. This prop- 

 erty has been used by Briicke as the means of detecting the presence of 

 free fatty acids, and, in all probability, the production of an emulsion in 

 the digestion of fats by pancreatic juice is due partly to this fact. When 

 subjected to dry distillation, acrolein is formed in conjunction with other 

 acrid and aromatic products. When fats are boiled with alkalies, soap 

 is produced by Union of the alkali with the fatty acid, forming a soluble 

 salt, or soap, while glycerin passes into solution. The glycerin may like- 

 wise be displaced by inorganic bases, such as lead, and glyceryl hydrate 

 or glyceryl alcohol (glycerin) is produced. This replacement of glyceryl 

 by other bases is termed saponification. The presence of glycerin may 



