xliv INTRODUCTION. 



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 matter, 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 a substance upon thin glass, the whole of its moisture must 

 first be expelled by evaporation ; otherwise the glass will certainly crack, and the experi- 

 ment 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 here add only a few 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, prin- 

 ciples, 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 proteine compounds from fatty matters, c. It al.so 

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

 plants is often imbued. Its action has been proposed to distinguish an animal from a 

 vegetable organism, and will yet probably prove to be of value in this respect. 



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

 renders also 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 (PI. 7. figs. 1 c and 3 c). In a few instances, how- 

 ever, 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 (Petten- 

 kofer's test). If the body contain lime (except already as sulphate), the acicular 

 crystals of the sulphate (PL 10. fig. 16) are produced. 



4. Muriatic acid with heat colours the proteine compounds. 



5. Acetic acid brings into view the nuclei of animal cells and tissues, dissolves many 

 salts, &c. 



6. Dilute nitric acid (20 per cent.) coagulates albumen, renders unstriped muscular 

 fibre-cells very distinct, c. Strong acid by boiling removes all but the cellulose from 

 woody fibre. 



