CHEMICAL REACTIONS. xxxvii 



objects whose morphological elements have become altered by individual growth, &c., 

 histological analysis is of course useless ; and the manner in which these have acquired their 

 existing structure, can only be determined by tracing the gradual changes which their mor- 

 phological constituents undergo, from the earliest period of their existence to that at which 

 they form the object in question. This constitutes the study of development, or it might be 

 termed Histological synthesis. It can rarely be followed directly ; but can often be carried 

 out indirectly, by examining a number of the objects in all stages of then* development, and 

 comparing the changes undergone by their constituents. It requires special care in con- 

 troling the identity of the objects. 



C. CHEMICAL REACTIONS. 



We cannot too strongly insist upon the necessity of investigating these in the case of 

 all objects submitted to examination, the nature of which is at all doubtful; and this, 

 because in many instances the form or general appearance will afford no criterion by 

 which the nature may be determined. Judgment founded simply upon the form, or 

 upon the 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 the micro-chemical analysis, as the Germans style it (and the 

 term is very convenient), 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 only give here a brief 

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

 ducted, 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 upper 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 quan- 

 tities 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-bottles). 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 



