HOW TO WORK 



feel as confident of the nature of the substance as if an ultimate 

 analysis were made of it. 



Besides the ordinary uses to which they are applied, chemical 

 reagents are useful in removing certain components of a structure 

 which interfere with the demonstration of other constituents, in alter- 

 ing the character of certain tissues without dissolving them, as for 

 instance by increasing their transparency or opacity, or in modifying 

 the physical structure of textures in such a manner as to render it 

 more convenient to cut sections or to perform other chemical opera- 

 tions necessary for the demonstration of their structure. 



By an acquaintance with the behaviour of certain substances with 

 particular chemical reagents, and the application of this knowledge to 

 microscopical investigation, we are often enabled to distinguish pecu- 

 liarities of structure, to ascertain the chemical composition of minute 

 quantities of matter, and to demonstrate clearly the existence of com- 

 pounds with the greatest certainty, which would entirely escape our 

 observation if we subjected them separately to the most careful 

 chemical analysis, or to the most searching microscopical exami- 

 nation. 



The application of chemical analysis to microscopical inv.estiga- 

 tion, and the examination of crystalline forms in the microscope, has 

 thrown a new light upon the nature of many physiological changes 

 which are constantly taking place in living bodies in health, and has 

 enabled us to investigate more satisfactorily the modifications which 

 these processes undergo when influenced by circumstances interfering 

 with or counteracting healthy actions. 



2SO. Instances of the Use of Reagents. As an instance of the 

 great advantage of the application of a few simple tests to microsco- 

 pical investigation, I may refer to the different effects of ether upon 

 fat globules (which are so commonly found in different tissues) and 

 crystalline bodies composed of phosphate or carbonate of lime, which 

 sometimes resemble them so nearly in refractive properties, in form, 

 and in general appearance, as to have led to mistakes with reference 

 to their nature. The application of a drop of ether has no effect 

 whatever upon the latter, but instantly dissolves the former. Phos- 

 phate of lime is readily soluble in dilute acids, while fat is not 

 acted upon by these reagents. Various insoluble saline materials not 

 unfrequently prevent us from seeing the anatomical elements of 

 which a tissue is composed. A knowledge of the nature of these 

 often enables us very easily to remove them. Suppose for instance, 

 the saline matter consists of carbonates or phosphates of lime or 

 magnesia, we have only to add a drop of dilute acid which dissolves 

 hem completely. 



