xxxviii INTRODUCTION. 



under examination, perhaps from an insufficient lapse of time for the occurrence of diffusion 

 in the two liquids. To be positive, therefore, that the reagent has no action upon the object 

 when none is at first apparent, as much as possible of the liquid in which it is immersed 

 should be removed by blotting-paper ; or the liquid be gently driven off by evaporation ; or, 

 if the object be of sufficient size to ensure its not being lost, the thin glass should be removed, 

 and the whole, or as much as possible, of the liquid removed either by the blotting-paper 

 or evaporation. On then covering the object with the thin glass, and adding the re- 

 agent to the edge of the latter, there can be no doubt of its coming into contact with the 

 body ; and the result may be considered as decisive. 



Where the combined effects of a reagent and heat are required to be observed, the former 

 may be added as usual, and the slide placed upon the tin plate mentioned at p.xvi. until the 

 liquid boils, or the requisite amount of heat has been applied ; the object, of course, being 

 covered by thin glass. The slide must then be allowed to become perfectly cold before 

 being placed under the microscope, otherwise the heat might melt the balsam with which 

 the lenses of the object-glass are cemented together. The cooling is much facilitated by 

 placing the slide upon a plate or surface of metal ; we generally use the foot, or a part of 

 the stand of the microscope, for this purpose. 



The effect of a red heat is sometimes very desirable to be tested. This may be accom- 

 plished by exposing the object, placed upon a strip of platinum foil, a piece of thin glass or 

 of mica, to the flame of a spirit-lamp. The odour evolved should be noticed. If this be 

 ammoniacal, or resemble that of burnt horn, the body, if not crystalline, is probably of 

 animal nature, and certainly contains nitrogen. 



If the body consist solely of inorganic matter, or of oxalates, it will not be blackened by 

 the heat. If it consist partly of inorganic and partly of organic origin, it will be blackened, 

 and the inorganic matter will be left in the form of an ash. The alteration produced in the 

 form of the object by the heat should also be noted. 



In applying a red heat to substances upon thin glass, the whole of its moisture must first 

 be expelled by evaporation, otherwise the glass will certainly crack, and the experiment be 

 spoiled. The strip of platinum may be held by forceps ; and the thin glass or mica, upon 

 a curved piece of iron wire. We can add here a few only of the reagents, the action of 

 which it may be most desirable to obtain in determining the nature of a doubtful body. 

 Further particulars will be given under the heads of the various reagents, principles and 

 tissues, in the body of the work. 



1. Solution of caustic potash (especially when heated). The cell- walls of plants are not 

 greatly affected ; they retain their primitive form, only becoming somewhat swollen, whilst 

 animal substances are mostly dissolved; chitine, however, is unaltered. The solution also 

 possesses a remarkable power of separating many animal structures into their component 

 cells, &c. When cold, it separates all proteine compounds from fatty matters, &c. It also 

 removes the foreign compounds with which the cellulose of the epidermal structures of 

 plants is often imbued. 



2. Solution of iodine (in water) dyes most animal and vegetable substances brown ; ren- 

 ders also carbonate of lime brown ; colours starch, certain cell-walls of vegetables, amyloid, 

 the amylaceous bodies of the human brain, &c., blue. 



3. Sulphuric acid, when added to the external coat or cell- wall of plants (cellulose) dyed 

 with iodine, renders it blue or purple. In a few instances, however, where cellulose exists 

 in animal tissues, the same blue colour is produced ; but in these there is real animal matter 

 also, recognizable by its appropriate tests. When added to bile or proteine compounds 

 mixed with solution of sugar, it renders them red (Pettenkofer's test). If the body contain 

 lime (except already as sulphate), the acicular crystals of the sulphate are produced. 



