xxxvi INTRODUCTION. 



collapsed, whilst the former retains its shape ; this is the ordinary occurrence in young vege- 

 table cells. The exosmotic effects of the chloride of calcium should be looked for soon after 

 its addition to the object, particular care being taken that it conies into contact with the 

 object ; for when solid or semisolid bodies are macerated for a long time in the saline solu- 

 tion, they will become contracted, and globules of sarcode will escape from them ; but we 

 believe that in all these cases there really exists a cell-wall, or a structure corresponding to 

 it ; hence by solid or semisolid bodies, we must be understood to mean those which differ 

 from cells according to the characteristic action of exosmose. 



It must be remembered that solution of chloride of calcium is a highly refractive liquid ; 

 hence it frequently renders globules so transparent, that they are almost or completely in- 

 visible, and thus apparently dissolves them ; sometimes also it really dissolves them. More- 

 over, many so-called unicellular vegetable organisms exhibit the contraction of the internal 

 cell-wall or primordial utricle, from long maceration in water only, as is so frequently seen 

 in the Desmidiaceae " mounted " in water. An aqueous solution of iodine is also frequently 

 useful in bringing to light the existence of an inner cell-wall, especially in vegetable struc- 

 tures, causing it to become wrinkled and collapsed. 



Cells have not the tendency to fuse together or adhere to each other, which globules of 

 sarcode or other glutinous solid or semisolid substances have. 



If the object be brittle, crushing it will sometimes show its internal structure, by allowing 

 the examination of the margins of the fragments. 



Spherical or rounded solid bodies, when immersed in water or other liquids of low re- 

 fractive power, generally present a much less distinct black margin than cellular bodies, or 

 those with membranous walls. 



The determination of the contents of an object furnished with an outer coat, must be 

 made according to the foregoing indications. The contents often consist of liquid in which 

 are suspended molecules and granules. If these exhibit molecular motion, the material in 

 which they are suspended must be liquid. It sometimes becomes a question, whether a 

 body enclosed within another is central or lateral. This is readily determined by causing 

 the body to revolve by inclining the stage of the microscope, when, if central and fixed, the 

 enclosed body will retain this position j and if it be less than the cavity of the enclosing 

 structure, positive indication will be afforded that the latter is solid, or at least that it does 

 not consist simply of an outer coat with liquid contents and the enclosed body. But if it 

 be attached to the inner wall of the enclosing structure, the eccentricity of its motion whilst 

 revolving will be evident. 



The contents of microscopic bodies are frequently rendered distinct by the addition of 

 reagents, and in some cases can alone be distinguished by their use ; thus the nuclei of ani- 

 mal cells are at once made evident by the addition of acetic acid, &c. 



We frequently have to decide whether the interior of an object is solid or tubular. If it 

 consist of a firm substance,, drying it, if in liquid, will cause the evaporation of the liquid or 

 other contents, and the entrance of air. A section of it will also show whether it is solid or 

 hollow. The effects of crushing it should also be observed. 



B. HlSTOLOGICAL ANALYSIS. 



This consists in the resolution of the object into its component morphological elements, 

 and is usually effected by subjecting it to the action of various chemical reagents, continued 

 maceration, &c. It must never be attempted if inorganic matters be present in quantity, 

 until these have been previously removed. The reagent used should be one which 

 exerts a solvent action upon the substance of which the object is composed, the action 

 being interrupted at a certain stage by the addition of water, &c. In regard to those 



