276 THE METABOLISM OF FAT 



Palmitin, 8 H 6 (0 16 H 81 2 ) 3 , melts at 50'5-66. Olein, C 3 H 5 (C 18 H 33 2 ) 3 , 

 melts at 5. The neutral fats are colourless or yellowish, and 

 in the pure state are without smell or taste. They are lighter than 

 water and insoluble in it. Soluble in boiling alcohol, they separate 

 out on cooling and generally in the crystalline form. They are 

 soluble in ether, benzol, and chloroform ; form emulsions with gum or 

 egg white, and very lasting fine emulsions with soap solutions. They 

 are not volatile, boil at about 300 0., undergoing partial decomposi- 

 tion, and burn with a luminous smoky flame. Neutral fats on being 

 strongly heated yield the powerfully smelling vapours of acrolein. 



C 8 H 5 (OH) 3 -2H 2 = 3 H 4 0. 



On hyd ration by the lipase of the pancreas, or by similar enzymes 

 obtained from other animal organs, or from plants, or by steam, the fats 

 undergo decomposition as represented by the equation C 3 H 5 (OR) 3 

 + 3H 2 = C 3 H 5 (OH) 3 + 3HOK. On boiling fat with a not too concen- 

 trated solution of potash, or better, an alcoholic solution of potash, 

 it decomposes with the formation of soaps. Fat on becoming 

 rancid decomposes into fatty acid and glycerine, the fatty acids are 

 subsequently oxidised to lower volatile fatty acids with unpleasant 

 odours. Air and light can bring about this change even in the 

 absence of micro-organisms, which are the usual cause. All com- 

 mercial fats contain fatty acids. 



To determine the existence of fat, the tissue in question is 

 extracted with ether, the ether evaporated off, and some of the 

 residue tested for neutral fat by the acrolein test. Another part 

 of the residue is dissolved in a neutral alcohol- ether mixture which 

 has been rendered faintly alkaline and coloured with phenol-phthalein. 

 If the red colour disappears, fatty acid is present. The fatty acid 

 can be saponified by a hot solution of alcoholic potash. After 

 saponification water and ether can be added," and the neutral fat 

 dissolved in the ether, and some of the soap in the water. The 

 soap-water can be decomposed by mineral acid, and then an oily 

 layer of fatty acid will float out. 



In the quantitative estimation of fat the tissue must be dried 

 and ground down to the finest degree, and even then ether will 

 not extract the whole of the fat. Pfliiger and Dormeyer digest with 

 pepsin and HC1 before extraction, while Rosenfelcl boils the sub- 

 stance in-alcohol for 30', extracts with chloroform 6 hours, and then 

 repeats both processes. The residues of the alcohol and chloroform 

 extracts are taken up with ether and finally weighed. This method 

 gives 40 per cent, more than the method of Dormeyer. The boiling 

 alcohol, however, renders nitrogenous matter besides the fat of the 



