FATS. 247 



with small quantities,, we obviously cannot rely on the acid reaction, 

 or on the formation of glycerine ; in such cases the simplest method 

 is to obtain an ethereal extract of the alcoholic extract to which a 

 little acetic acid had been added, and then, by digestion with water, 

 to separate the residue of the ethereal solution from other sub- 

 stances. The remaining fat is then to be dissolved in alcohol, and to 

 be treated with an alcoholic solution of acetate of lead. If the 

 addition of ammonia give rise to no precipitate, it is a proof that 

 the solution contains no free fatty acids, but only salts of oxide of 

 lipyl. 



Free fat in the animal fluids, tissues, and cells, is most commonly 

 and, indeed, most satisfactorily detected by the microscope ; the 

 vesicles in which fat ordinarily appears, present so characteristic 

 an appearance, that when they have been seen for a few times under 

 the microscope, they can hardly be confounded with anything else; 

 the more consistent fat, containing little olein, sometimes, however, 

 occurs in nodular, sausage-shaped, and only faintly-transparent 

 clumps, which cannot so readily be recognised as fat. In these 

 cases, chemistry must come to the aid of microscopic investigation, 

 as, for instance, where the fat-vesicles in cells are so minute, that, 

 with the highest magnifying powers, they appear as mere dark 

 points or granules. Many histologists now maintain that these 

 points and aggregate granules may be very readily distinguished 

 under the microscope, by their solubility in ether ; but the extrac- 

 tion of the fat from the cells by ether, is by no means easy, for its 

 rapid evaporation under the microscope, renders it very difficult, if 

 not impossible, to observe the individual cells. Before making 

 our observations we must, therefore, repeatedly pour a little ether 

 on the object, and allow it again to run off, or if we have fine 

 sections of tissue, we may digest them in ether. Unfortunately 

 however, the cells and other histological elements are often so 

 distorted by ether, that even after long maceration in water, an 

 accurate observation is no longer possible ; and it is nearly the 

 same in most cases with alcohol, by which, however, well-prepared 

 sections of many parts, as, for instance, nerve-fibres, may often 

 have their fat thoroughly removed. Moreover, alkalies cannot be 

 advantageously applied to the partial saponification of these fats, 

 since they often dissolve albuminous parts much sooner than the 

 fats. We shall see, in a future part of this work, that some 

 histologists believe that they have found fat-granules in tissues 

 which have been hitherto regarded as utterly devoid of fat : and 



