108 SOLID FATTY ACIDS. 



is soluble in boiling ether, while the margarate of lead is insoluble, 

 we have an easy means of separating the two salts. The marga- 

 rate of lead must then be decomposed by an alkaline carbonate, 

 and the resulting alkaline salt by a stronger acid. The margaric 

 acid which is thus separated may be further purified by solution in 

 hot alcohol. 



Tests. From the properties, as well as from the mode of pre- 

 paring this acid, we perceive that it can only be distinguished from 

 other similar acids when it is perfectly free from any admixture with 

 them : we may derive some information on this head from its 

 boiling point ; but it is only by an elementary analysis that we 

 can arrive at any certain conclusion. In the investigation of small 

 quantities, when a separation or an analysis is out of the question, 

 we must trust solely in a microscopical examination, which, how- 

 ever, in this case yields by no means such uncertain results as 

 is generally supposed. 



Physiological Relations. 



Occurrence. It has already been remarked that margaric acid 

 is the principal constituent of most animal fats ; but this acid is 

 here ordinarily combined with the hypothetical haloid base, oxide 

 of lipyl) which, in its separation from this and similar acids, is con- 

 verted into the well-known body, glycerine. Of margarin itself we 

 shall speak in a future part of this volume, and we shall con- 

 sequently defer for the present all remarks on the physiological 

 function of margaric acid and its organic salts. But margaric 

 acid occurs both in a free state and in combination with alkalies in 

 most of the animal fluids, with the exception of urine ; being free 

 in acid fluids, and in a state of combination in those with an alka- 

 line reaction ; it is always accompanied by oleic acid or its salts. 

 Its presence in the saliva, in the blood, in exudations of all kinds, 

 in pus, and in the bile, is so easily recognised, that it is unneces- 

 sary to quote authorities regarding its existence in these fluids ; 

 moreover, in our remarks on these fluids we shall return to this 

 subject. We will here only remark that it may also be discovered 

 in the solid excrements after the use of vegetable food, and that 

 it occurs in considerable quantity in dejections which have been 

 caused by purgatives or mineral waters. As already mentioned, 

 we must here always have recourse to the microscope, by which, 

 independently of any chemical process, free margaric acid may 

 often be detected in acid pathological fluids ; thus, in acid pus 

 discharged from what are termed cold abscesses, or in pus in 



