212 HOW TO WORK 



to stand still for a short time, when certain changes occur in 

 some of its constituents, which lead to the precipitation of some 

 bodies in a crystalline form, as for instance, uric acid, or -triple 

 phosphate. In other cases it becomes necessary to add some 

 reagent before the crystals are thrown down, while not unfrequently 

 a long and often complicated chemical analysis is necessary if 

 substances which were previously held in solution, are to be 

 obtained in a crystalline form. The addition of water in some 

 cases causes the most rapid crystallisation, especially when the 

 crystallisable material is dissolved in viscid organic matter, as when 

 water is added to blood, in order to obtain blood crystals. In- 

 stead of water, in other instances, alcohol, ether, or chloroform, in 

 which the crystals may be much less soluble than in water, is to 

 be preferred. 



Crystalline substances which are dissolved in animal fluids, 

 may often be separated in a perfectly pure state by the addition 

 of another fluid in which they are not so readily soluble. This 

 last should be added very gradually, to allow time for the forma- 

 tion of the crystals, otherwise an amorphous precipitate alone 

 results. Many organic substances soluble in alcohol, may be 

 crystallised by the addition of ether, while some are precipitated 

 from their solution in water, by the gradual addition of 

 alcohol. 



312. Influence of various Constituents upon the Crystallisation. 

 In many instances, it is exceedingly difficult to separate some 

 crystalline bodies from other constituents by which their solubility is 

 much increased, and crystallisation is often prevented. The extrac- 

 tive matters of blood, and of many organic fluids, exert this influence 

 in a marked degree, and it is only of late years that several new 

 bodies of definite chemical composition have been isolated. Creatine 

 and creatinine may be instanced amongst the number, for these were 

 not very long ago included under the indefinite term " extractives." 

 Certain colouring matters of definite composition have also been sepa- 

 rated, and it is very probable that as the methods of analysis at our 

 disposal become improved, many new crystalline bodies will be isolated 

 from the extractive matters. A very small quantity of extractive matter 

 entirely prevents the crystallisation of urea, while the presence of 

 chloride of sodium or common salt favours the separation of this mate- 

 rial by forming with it a compound which readily crystallises in large 

 octohedral crystals even in the presence of extractive matters. The 

 existence of carbonic acid in excess may cause carbonate of lime, 

 triple phosphate, and other salts, to be held in solution. Excess of 

 alkali prevents the precipitation of uric acid, and excess of acid, that 



