444 PEACTICAL PHYSIOLOGY 



so as to remove the silver which it contains : the silver sulphide is 

 filtered off, and the filtrate evaporated till all the ammonia has been 

 expelled. It is then made strongly acid with phosphoric acid, and the 

 lactic acid, which is hereby liberated, is dissolved out by shaking it in 

 a separating funnel with ether (the method employed is the same as that 

 described for separating fats and soaps on p. 182). 



After extracting three or four times, the ethereal extracts are 

 combined and the ether evaporated away by placing on a water bath 

 heated to about 60 C., the flame underneath which has been ex- 

 tinguished. An acid syrup remains behind ; this is impure lactic acid. 

 In order to purify it, dilute three times with water, bring the resulting 

 solution to the boil, and then carefully add powdered zinc carbonate until 

 the reaction is neutral. Filter. Evaporate the filtrate to small bulk, 

 and add an equal bulk of spirit and allow to stand, when zinc sarco- 

 lactate will crystallise out (Fig. 271). The zinc salt is filtered off, 

 washed several times with spirit, dissolved in water, 1 and the zinc 

 separated by passing a stream of sulphuretted hydrogen through 

 the solution. The zinc-free filtrate is then freed of water by evapora- 

 tion, when the lactic acid is obtained as a syrup. 



Chemical Beactions and Tests. The third member of the fatty acid 

 series is called Propionic acid, and has the formula CH 3 CH 2 COOH. If 

 one of the hydrogen atoms of the central methyl group of this be 

 replaced by a hydroxyl radicle, we obtain lactic acid, which has, there- 

 fore, the formula CH 3 CH(OH) . COOH. By examining this formula 

 more closely it will be seen to contain an asymmetric carbon atom ; that 

 is to say, an atom of carbon whose affinities are occupied by four 

 dissimilar radicles. Thus : 



H 



i 



H 3 C-C-COOH 



OH 



Now, it will be remembered when studying carbohydrates that, when 

 such a condition as this exists, the substance has stereochemical 

 properties, i.e. that it rotates the plane of polarised light, and, moreover, 

 that it presupposes the existence of two bodies with the same structural 

 formulae, and consequently possessing the same chemical reactions, but 

 differing from one another in their stereochemical behaviour, one 

 rotating to the right, the other to the left. It also presupposes that 

 there may exist a body which is optically inactive, being composed of 



1 The watery solution should be evaporated until the crystals of zinc sarco- 

 lactate begin to appear, this being ascertained by examining a drop under the 

 microscope. 



