FATTY SUBSTANCES. 135 



matters as combinations of glycerine playinor the part of a uase with 

 particular acids ; these combinations, analogous to salts if their con- 

 stitution be merely considered, are generally mixed together u; oils 

 and fats ; thus the union of stearic acid and glycerine forms stearine, 

 which is fusible at the temperature of about 62" cent. (144° Fahr.) 

 Stearic acid melts at 72° cent., (162" Fahr.,) oleine remains fluid at 

 4° cent., (24° Fahr.,) and oleic acid is liquid. An oil is, therefore, 

 by so much the more consistent as a larger quantity of solid fatty 

 acid enters into its composition, and it is, on the contrary, by so 

 much the softer and more liquid as this acid is itself more fluid. The 

 wax of the Myrica cerifera, for example, is sufficiently hard to be 

 reduced to powder, and is almost entirely formed of stearine. In 

 the fluid vegetable oils oleine always predominates. It is easy to 

 separate these different fatty compounds from one another. 



Besides the solid and li(juid acids which are obtained from fatty 

 substances, there are others known which are volatile. 



Fatty bodies absorb oxygen from the air. This absorption is at 

 first extremely slow, scarcely appreciable ; but once begun, it goes 

 on with great rapidity ; so rapidly, indeed, that if a large surface be 

 exposed to the air, if, for example, a quantity of rags or tow be im- 

 pregnated with oil, the mass may take fire. The consequence of 

 this oxidation is always a thickening of oil, and there are some which 

 become completely solid in its course ; these are designated by the 

 title of drying oils, and are in particular request for the manufacture 

 of varnishes. Nut oil which has remained long exposed to the air 

 acquires the consistence of jelly, and its unctuous properties have so 

 entirely disappeared that it no longer stains paper. 



The alteration which fatty substances undergo in contact with air 

 and moisture is still more remarkable. The oils which are inodorous 

 and without taste soon acquire under these circumstances a strong 

 smell and a disagreeable flavor. Fleshy fruits which contain a large 

 quantity of oil, such as the olive and the oleaginous seeds, when 

 moistened suffer true fermentation, the result of which is the sepa- 

 ration of the fatty acids from the glycerine. 



Oils subjected to the action of a high temperature are also greatly 

 modified. The glycerine which they contain is decomposed, and 

 gives rise to various pyrogenous products : stearic acid is changed 

 into margaric acid, and oleic acid into sebacic acid, a crystallizable 

 volatile acid which is soluble in hot water. 



The fatty substances of plants are principally accumulated in the 

 fruit, and particularly in the seed. In the herbaceous parts they are 

 less abundant, less perfectly elaborated. Oils appear to be included 

 in the vegetable tissue under the form of globules, or minute drops. 

 In such an oily seed as the common almond, when it is growing, we 

 perceive that the cellular tissue is in the first instance full of a col- 

 orless and transparent fluid ; but as the seed advances, each cell is 

 seen to become filled with numbers of little oil globules which in- 

 crease continually in size and number until the kernel is ripe ; there 

 is at the same time a quantity of azotized matter deposited in the 

 midst of the liquid, which disturbs its transparency ; it is this depos- 



