774 THE POPULAR SCIENCE MONTHLY. 



origin) may be distilled or simply evaporated away like -water or 

 alcohol, and leave no residue. The fixed oils similarly treated are 

 dissociated more or less completely. 



Otherwise expressed, the boiling-point of the volatile oils is below 

 their dissociation-point. The fixed oils are those which are dissociated 

 at a temperature below their boiling-point. 



My object in thus expressing this difference will be understood 

 upon a little reflection. These volatile oils, when heated, being dis- 

 tilled without change are uncookable ; while the fixed oils if similarly 

 heated suffer various degrees of change as their temperature is raised, 

 and may be completely decomposed by steady application of heat in a 

 closed vessel without the aid of any other chemical agent than the 

 heat itself. This " destructive distillation " converts them into solid 

 carbon and hydrocarbon gases, similar to those we obtain by the de- 

 structive distillation of coal. 



If we watch the changes occurring as the heat advances to this 

 complete dissociation-point, we may observe a gradation of minor 

 or partial dissociation proceeding gradually onward, resembling that 

 which I have already described as occurring when sugar is similarly 

 treated (see No. XIII of this series). 



But in ordinary cooking we do not go so far as to carbonize the 

 fat itself, though we do brown or partially carbonize the membrane 

 which envelops the fat. What, then, is the nature of this minor dis- 

 sociation, if such occurs ? 



Before giving my answer to this question, I must explain the 

 chemical constitution of fat. It is a compound of a very weak base 

 with very weak acids. The basic substance is glycerine, the acids 

 (not sour at all, but so named because they combine with bases as the 

 actually sour acids do) are stearic acid, palmitic acid, oleic acid, etc., 

 and bear the general name of fatty acids. They are solid or liquid, 

 according to temperature. When solid, they are pearly, crystalline 

 substances ; when fused, they are oily liquids. 



To simplify, I will take one of these as a type, and that the one 

 which is the chief constituent of animal fats, viz., stearic acid. I have 

 a lump of it before me. Newly broken through, it might at a distance 

 be mistaken for a piece of Carrara marble. It is granular like the 

 marble, but not so hard, and, when rubbed with the hand, differs from 

 the marble in betraying its origin by a small degree of unctuousness, 

 but can scarcely be described as greasy. 



I find by experiment that this may be mixed with glycerine with- 

 out combination taking place ; that when heated with glycerine just to 

 its fusing-point, and the two are agitated together, the combination is 

 by no means complete. Instead of obtaining a soft, smooth fat, I ob- 

 tain a granular fat, small stearic crystals with glycerine among them. 

 It is a mixture of stearic acid and glycerine, not a chemical compound ; 

 it is stearic acid and glycerine, but not a stearate of glycerine. 



