316 PEACTICAL PHYSIOLOGY 



at a time to a solution of the acids in alcohol. Each addition produces a pre- 

 cipitate, which is filtered off and treated with dilute hydrochloric acid and ether. 

 The hydrochloric acid decomposes the lead salt, and the liberated fatty acid goes 

 into solution in the ether. This process is called fractional precipitation, and 

 the higher the melting point of the acid the more easily is it precipitated by the 

 lead acetate. 



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

 fat gives an important reaction, depending on the glycerine which it 

 contains. This is called the acrokin 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. 



Emidsificatim. 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 Experiment VII. 

 into two parts ; to one of these add a little mucilage or egg-albumin 

 and shake, and note that the emulsion " stands " much longer than that 

 to which no suspending medium has been added. 



LECITHINS. 



Soluble in acetone, and otherwise very closely related to the 

 fats, is a group of bodies called lecithins. The lecithins are present 

 in greater or less amount in all the cells of the body. Most plentifully, 

 they occur in the envelope and stroma of the red-blood corpuscles, in 

 nervous tissues and in bile. They are also found in plants. Chemically 

 these bodies consist of a glycerine molecule, two of the hydroxyl groups 

 of which are combined with fatty acid and the remaining one with 



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. 



