WITH THE MICROSCOPE. 213 



of phosphate of lime. Fatty matters dissolve cholesterine, and serum 

 possesses the power of retaining small quantities of both the latter 

 substances in solution. Some crystalline bodies which are soluble 

 at the temperature of the body, crystallise when the solutions con- 

 taining them are cooled thirty or forty degrees. The effect of dilu- 

 tion in retaining crystals in solution, need scarcely be alluded to. 

 Hence, before the presence of many substances can be detected by 

 microscopic examination, certain chemical operations are required in 

 order to separate them from their combinations in the animal body, 



or for the removal of other substances which interfere with their 

 crystallisation. 



313. Separation of Crystals from Animal Substances. From 

 what was stated in the last section, it follows that in many instances 

 this is a matter of some difficulty. Not unfrequently, if not veiy 

 soon separated from the fluid in which they were formed, the crystals 

 again undergo solution or become decomposed. If the crystals are 

 not very soluble, the supernatant fluid, or mother-liquor, may be 

 poured off, the crystalline deposit washed with ice-cold water, and 

 subsequently dried on filtering paper over sulphuric acid without the 

 application of heat. 



If the crystals will not bear the application of water, as much of 

 the fluid as possible must be poured off, and the remainder absorbed 

 with bibulous paper, or they may be placed upon a porous tile, and 

 dried over sulphuric acid in vacuo. In many instances we are 

 enabled to wash the crystals with water, holding a little acid or alkali, 

 or some alkaline salt, in solution, or with alcohol, ether, or some 

 other fluid in which we know them to be quite insoluble. 



In cases in which crystals insoluble in water are deposited in 

 animal solids,, they may be separated by agitation, when, being, 

 heavier than the water, they subside to the bottom, and the lighter 

 animal matter may be removed by forceps, or if in a very minute state. . 

 of division, poured off with the supernatant fluid. In other cases, it may 



be separated by straining, while the crystals are washed through muslin. 



T j 



314. Of obtaining Crystals for Examination. In order to 



accustom himself to the necessary manipulation required in the 

 process, the student may evaporate a solution of common salt upon 

 a glass slide, and when it has become sufficiently concentrated it may 

 be covered with a small piece of thin glass, and allowed to cool. 

 When cold it may be subjected to microscopical examination. Beau- 

 tiful cubes of chloride of sodium will be observed, pi. XLII, 

 fig. 270. Crystals of several salts may be made in the same simple 

 manner, and from an attentive examination of them, much may be 

 learned. Phosphate of Soda, Phosphates of Soda and Ammonia, 



