CHEMICAL REACTIONS. xliii 



mere inspection of an object, therefore, will illustrate the abuse and not the proper use of 

 the microscope. The quantitative and ultimate analysis of substances cannot be made in 

 any manner by the aid of microscopic manipulation ; but the qualitative analysis, or the 

 study of the action of chemical reagents upon the object or substance by the aid of the 

 microscope, or micro-chemical analysis, may be undertaken with the prospect of almost 

 certain success, in most cases at least, in ascertaining the proximate chemical composition. 



The characteristic reactions or tests for the various proximate principles are given in 

 this work under the respective heads of those substances ; and we can here give only a 

 brief sketch of the manner in which the micro-chemical analysis of a substance may be 

 conducted, and without which its microscopic investigation must be imperfect and of little 

 or no value. 



The first point to be attended to is, to ensure, as far as possible, the freedom of the 

 object from foreign admixtures. Thus, if it should have been found in an animal or vege- 

 table liquid, it must be carefully washed, either in a watch-glass or upon a slide whilst 

 covered with thin glass. The former is readily accomplished : the substance being placed 

 in a watch-glass, water or other solvent of foreign matters is added ; the whole is then 

 set aside, to allow of the subsidence of the substance, and the supernatant liquid removed 

 by a pipette. If the body or the particles be very minute, it or they must be placed upon 

 a glass slide, and covered with thin glass ; the latter should then be pressed, so far as is 

 possible without crushing the particles, but sufficiently to fix them, and a small piece of 

 coarse white blotting-paper placed upon the surface of the slide, so as to touch the 

 edge of the liquid ; capillary attraction will cause the liquid to be absorbed by the paper. 

 Small quantities of water, or other proper solvent, are then added by small portions from 

 the end of a glass rod to the opposite edge of the liquid confined by the thin glass. Thus 

 a current will be set up, and the newly added liquid will be absorbed by the blotting- 

 paper, washing in its course the particles confined between the two glasses. The current 

 will be regulated by the quantity of liquid added, and the facility with which the paper 

 absorbs it. 



"When the body has been washed, the effects of the various reagents may be examined, 

 by the addition of them in small quantities from the conical stoppers of the test-bottles 

 (see Test-box, p. xxvii). The test-liquid being applied to the edge of the liquid in which 

 the body is immersed, gradually mixes with it, and the effects produced may be watched 

 step by step. If a solvent or other action is seen to take place, the result is decisive ; 

 but if no action be evident, it must be remembered that the reagent added may not have 

 reached the object 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 reagent 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 

 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 brass table mentioned at p. xxviii 



d 



