FATS AND OILS. WAXES. LECITHINS 



179 



syphon tube is attached ; its other end is fused to the narrow piece 

 through which it passes and opens just above the angular extremity. 

 The narrow end of this tube is fastened into the flask ; the condenser 

 is attached to the wider end. Ether is placed in the flask which is 

 gently heated. The volatilised ether passes through the side tube and 

 reaches the condenser. The condensed drops fall upon the thimble 

 and cover it. When completely covered the ether is syphoned off 

 and returns to the flask and the process is repeated. It remains to 

 distil off the ether from the flask, dry at 100 and weigh. The differ- 

 ence in weight gives the amount of fat in the known weight of tissue. 

 This method gives comparatively good results ; other substances 

 besides fats are extracted from the tissue and some of the fat, present 

 inside the cells, is not extracted. It is now more usual to estimate the 

 fat as fatty acid, see page 60 1. 



Analysis. 



The natural fats consist of a mixture of the glyceryl esters of the saturated 

 fatty acids, butyric, caproic, palmitic and stearic, of the unsaturated fatty, acids, 

 oleic, linoleic and others and also of hydroxy fatty acids. Free fatty acids are 

 present in small quantities and increase in amount as the fat is kept. The 

 various fats and oils have a fairly constant composition so that by determining 

 the amounts of the various constituents it can be identified. The following 

 six analyses are usually made : 



(1) the acid value, i.e. the amount of potassium hydrox'de in mgm. re- 

 quired to neutralise the free fatty acid in i gm. of fat ; 



(2) the saponification value, i.e. the amount of potassium hydroxide in 

 mgm. required to saponify i gm. of the fat ; 



(3) the iodine value, i.e. the amount of iodine in gm. absorbed by 100 gm. 

 of the fat ; 



(4) the Reichert-Meissl or Reichert-Wollny value, i.e. the amount of 

 potassium hydroxide in c.c. of -iN required to neutralise the volatile fatty 

 acids in 5 gm. of the fat ; 



(5) the Hehner value, i.e. the amount of non-volatile and insoluble fatty 

 acids (and unsaponifiable matter) present in 5 gm. of the fat ; 



(6) the acetyl value, i.e. the amount of potassium hydroxide required to 

 combine with the acetic acid in i gm. of fat, which has been acetylated. 



The values of some of the commoner fats are given in the accompanying 

 table : 



