ELEMENTARY CHEMICAL PHYSIOLOGY 223 



B. Add a piece of fatty acid to some half-saturated solution of sodium 

 carbonate, and warm ; the fatty acid dissolves, carbon dioxide is liberated, 

 and a solution of soap is obtained. Neutral fat is insoluble in cold sodium 

 carbonate solution. 



C. Press out some fatty acid between filter paper until it is dry, and apply 

 the acrolein test as described in Experiment V. (see below). The result is 

 negative. 



D. To a solution of soap add : (a) a few drops of a solution of calcium 

 chloride a white precipitate of a calcium soap falls down ; (6) some lead 

 acetate solution a white precipitate of lead soap falls down (lead plaster). 



The fatty acids prepared by the above method mainly consist of a mixture 

 of palmitic, stearic and oleic. To separate these from one another, advantage 

 is taken of the fact that they differ in the readiness with which they form 

 salts (soaps) with lead acetate (p. 299). 



Besides these reactions of the fatty acid produced from it, neutral 

 fat gives an important reaction, depending on the glycerol which 

 it contains. This is called the acrolein reaction. 



EXPERIMENT V. Place a small piece of fat in a thoroughly dried 

 test tube, add to it three or four times its bulk of acid potassium 

 sulphate, 1 and heat. A pungent vapour of acrolein 2 is given off, 

 which blackens a piece of filter paper which has been dipped in 

 ammoniacal silver nitrate solution. This reaction demonstrates 

 that the vapour acts as a reducing agent. 



Emulsification. -When oil is mixed with water it floats to the 

 surface, but when a soap is present in solution in the water the oil 

 globules remain suspended, and an emulsion results. This is more 

 permanent if some suspending medium such as mucilage be added. 



EXPERIMENT VI. In one test tube (a) place some soap solution ; 

 in another (6), some water. To each add some neutral olive oil 

 and shake. Allow to stand, and note that a remains emulsified, 

 b does not. 



EXPERIMENT VII. Place some rancid oil (i.e. containing free 

 fatty acid) in a test tube, add some weak caustic potash solution 

 and shake ; an emulsion forms, soap being formed by the alkali 

 combining with the fatty acid. 



EXPERIMENT VIII. Divide the emulsion produced in Experi- 

 ment VII into two parts ; to one of these add a little mucilage or 

 egg-albumen and shake, and note that the emulsion persists 

 much longer than that to which no suspending medium has been 

 added. 



Lipoids 



In addition to the neutral fats there exists in the different tissues 

 in very varying amount a series of substances, many of them ill- 

 defined, which possess many of the characteristics of fat and 

 to which the name lipoid has been given. These lipoids have not 



1 Commercial acid potassium sulphate is often impure and gives a pungent 

 reducing vapour by itself. It is well, therefore, to make a preliminary test 

 with the crystals alone. The impure salt can be readily purified by 

 crystallisation. 



2 Acrolein is the aldehyde of allyl alcohol and has the formula 



CH 2 = CH - CHO. 



