222 Chemistry. [Lecture 3 1 . 



as is sometimes practised with olives, and more 

 especially with the animal oils. The water in 

 this case dissolves the matter of which the cells 

 are composed, and sets the oil at liberty. 



" It deserves attention," Dr. Thomson re- 

 marks, " that the only part of vegetables in which 

 fixed oils are found is the seeds of bicotyledi- 

 nous plants." In animals they are usually depo- 

 sited in the most copious quantity in the adipose 

 membrane, as in the lard of hogs, the tallow of 

 cattle, and the blubber of the whale. 



Fixed oils are at present considered as com- 

 posed only of hydrogen and carbon. By the 

 analysis of Lavoisier 100 parts of olive oil con- 

 sisted of about seventy-nine parts carbon, and 

 twenty-one hydrogen. 



Some of these oils have the property of dry- 

 ing easily without losing their transparency, such 

 as linseed oil, nut oil, poppy oil, and hempseed 

 oil. This property is increased by the addition 

 of litharge (an oxide of lead) which is partly re- 

 duced by the process, whence it follows that this 

 property is increased by the absorption of oxygen, 

 or by taking away part of the hydrogen. 



The mild alkalies, carbonate of potass, soda, 

 &c., have little effect upon these oils; but the 

 caustic, or pure alkali, has a considerable at- 

 traction for them, and unites with them into a 

 compound called soap. The caustic alkali gives 

 the soap an attraction for water, and penetrates 

 the oily matter, which we want to wash out. 



