320 PRACTICAL PHYSIOLOGY 



Shake gently. On settling, it will be seen that the chloroformic 

 solution becomes coloured blood red and afterwards purple, and the 

 sulphuric acid shows a green fluorescence. If the chloroformic solution 

 be moistened with water, as by pouring it into a moistened test tube 

 the colour disappears. (Salkowski's reaction.) 



EXPERIMENT XI. Dissolve some cholesterol in acetic anhydride, 

 and, after cooling, add some sulphuric acid (cone.). A play of colours 

 results. (Liebermann's reaction.) 



PROTAGON. 



This name is given to a crystalline substance containing phos- 

 phorous and nitrogen. It can be prepared from brain tissue by 

 various methods, but perhaps most simply by extraction, by means of 

 hot acetone, of a mixture of gypsum and brain tissue (see p. 319), from 

 which the cholesterol has been previously removed by treatment with 

 cold acetone. The hot extract, after filtration, deposits crystals of 

 so-called protagon on cooling. By fractional precipitation, or by 

 treatment with different solvents, protagon can be shown to be a 

 mixture of different lipoids, some of which contain large amounts of 

 phosphorus (sphingomyelin), whilst others are phosphorus free (phreno- 

 sin). Hydrolysis of protagon yields galactose, choline, and other 

 Imses. (See table, p. 321.) 



FAT VALUES. 



For many reasons it is important that the physiologist and hygienist should be 

 acquainted with the chemical methods used for distinguishing the various fats. 

 It is by an application of such methods that the physiologist has been able to 

 show, among other things, that forced feeding with a fat- rich diet (after previous 

 starvation) leads to the deposition in the tissues of fats very similar to those 

 contained in the food. By such methods, also, the hygienist is able to tell when 

 butter, for example, is of proper composition, for it is easy for the merchant to 

 substitute other fats (oleomargarine) for it. 



The following are the most important of these methods : 



I. Melting Point. The method for estimating this has been given on p. 277. 

 The absorbability of a fat from the intestine varies inversely with its melting 

 point, e.g. mutton fat with a melting point of 44-51C. is absorbed much more 

 slowly than is pig fat with a melting point of 36-46C. 



II. Specific Gravity. 



EXPERIMENT. Melt pieces of butter and of oleomargarine in two small evapora- 

 ting dishes, and drop the melted fats into alcohol at room temperature (15 C.). 

 The butter will sink, hut the oleomargarine will float, since it is composed of fats 

 of lower specific gravity than those of butter. 



III. Acid Value. This refers to the amount of free fatty acid which the 

 specimen of fat contains. When fats become rancid, the acid value rises con- 

 siderably. 



