110 Progress of Foreign Science. 



can never be dried by atmospheric exposure ; they retain more or 

 less water which renders them soft or gelatinous ; and if after dry- 

 ing them with heat, we put them into cold water, they diffuse and 

 dissolve. The causes of these differences are to be found; 1, in 

 the nature of the alkaline base ; 2, in that of the fat matter com- 

 bined with this base. 



1. Influence of the alkaline Base. If we saponify some of the same 

 fat body with potash and soda, it is constantly observed that the 

 soda soap is less soluble in cold water than the potash soap. 



Influence of the fat matter which is combined ivith the alkali. Oil 

 of olives, and particularly the less fusible animal fats, form, with 

 soda, soaps which are much harder, than the soda soaps from rape- 

 seed and animal oils ; and, secondly, these oils form, with potash, 

 much softer soaps, than those containing olive oil and the less fu- 

 sible fats. M. Chevreul thinks that his researches completely 

 explain these results. Let us consider, first of all, the action of cold 

 water on the soaps, or in other words, on the salts which the stearic, 

 oleic, and margaric acids form with soda and potash*. 



The stearate of soda may be considered as the type of the hard 

 soaps ; it appears to experience no action from ten times its weight 

 of cold water. The stearate of potash produces a thick mucilage 

 with the same proportion of cold water. 



The oleate of soda is soluble in ten times its weight of cold 

 water ; the oleate of potash forms a jelly with the double of its 

 weight of water, and a solution with four times its weight. It is 

 so deliquescent that'l 00 parts absorb, in an atmosphere saturated 

 with moisture, 162 parts of water, at the temperature of 12° C. 



The combinations of margaric acid with soda and potash, differ 

 from those of the stearic acid, only in the somewhat greater action 

 of water on them. The stearates, margarates, and oleates of the 

 same bases can combine together in all kinds of proportions. 



1. The soaps of human fat, and vegetable oils, are formed of 

 oleates and margarates, whose respective proportions are very 

 variable ; and the soaps are softer, the more oleate, and the less 

 margarate, they may contain. 2. The soaps of mutton suet, tal- 

 low, hog's lard, and butter, putting out of view the odorous salts 

 they may contain, are formed not only of margarate and oleate like 

 the preceding, but also of stearate ; and it is remarked that their 

 hardness is greater as the stearate predominates over the oleate. 

 On the other hand, his experiments having shewn that it is chiefly 

 the stearines which yield the stearic and margaric acids, and the 

 oleine which yields the oleic acid, it follows ; 1, that according to 

 the proportion of the stearine to the oleine, contained in the sapo- 



* He calls stearic acid, that which has the closest relations with the mar- 

 garic acid, but which differs from it, in melting only at 70° C and in contain- 

 ing less oxygen. 



