WITH THE MICROSCOPE. 211 



precipitate of sulphate of baryta, insoluble in strong acid and 

 alkalies, takes place, if sulphuric acid be present. The presence of 

 chlorides is detected by the addition of a little nitrate of silver 

 to a drop of the solution of the deposit in weak nitric acid. The 

 white precipitate of chloride of silver is insoluble in nitric acid, but 

 is dissolved by ammonia. 



The above will serve as examples of the method of detecting the 

 presence of different substances in a very minute quantity of matter. 

 The indications obtained in this manner are quite as valuable, and 

 may be relied upon with as much certainty, as if we were provided 

 with a very large quantity of material to work upon. In a single 

 drop of a composite solution, the presence of several different acids 

 and bases may be detected. 



310. New Method of Microscopical Analysis. Since I have been 

 in the habit of using glycerine as the basis of all my injecting 

 fluids and preservative solutions, I have employed it as the solvent 

 of all tests, and with the greatest advantages. The reactions are of 

 course slower, but much more perfect. Crystals can be formed 

 most readily by this process, and as the viscid solutions mix very 

 slowly, most perfect crystals even of substances which crystallise 

 with great difficulty in water, can be readily obtained. If glycerine 

 be added gradually to many solutions of crystallisable matter 

 crystals are deposited. The various tests may be dissolved in a 

 little water and then added to Price's glycerine. The iodine 

 reactions can often be obtained most satisfactorily by this mode of 

 proceeding. The solutions may be kept in the little glass bottles 

 described in p. 209. Very strong solutions of the nitric and 

 sulphuric acids cannot be obtained in glycerine, but it is seldom that 

 a stronger solution than one part of acid to five of glycerine is 

 required. If a very strong viscid solution of acetic acid be wanted, 

 the acid may be warmed with lump sugar in sufficient quantity to 

 make a fluid of the consistence of syrup. Glycerine may be 

 employed as the universal medium for the examination, preservation, 

 and qualitative analysis of microscopic objects. It need- scarcely be 

 said that glycerine and syrup are miscible, so that the viscidity of any 

 fluid can be readily increased by the addition of sugar to it. 



Of obtaining Crystalline Substances from the fluids and 

 Textures of Organism. 



311. Formation of Crystals. Some crystalline bodies are 

 deposited from their solution in animal fluids by simple evapora- 

 tion ; others, less soluble, may be deposited by allowing the fluid 



p 2 



