MICROSCOPIC ANALYSIS. INTERNAL STRUCTURE. xli 



of an object, i. e. -whether it is solid or cellular, &c. ; and where an object is composed of 

 an aggregation of similar parts, our remarks must be applied to these individually. 



The first question arising is whether a transparent object is solid or semisolid and homo- 

 geneous, or whether it represents a cell, i. e. has an outer membrane or cell-wall and con- 

 tents of a different nature. When objects possess an outer coat, its two margins are 

 sometimes easily distinguishable on examination by transmitted light, especially when its 

 thickness is considerable. But when the outer coat is thin, these are difficult to distinguish ; 

 recourse must then be had to other means than simple inspection ; and these will vary 

 according to the nature of the object, and especially the softness of its cell-wall. Some- 

 times crushing it may show clearly that the contents consist of a liquid with numerous 

 molecules and granules, and that the cell-wall is thin and membranous ; for the subsequent 

 addition of water may separate and render both distinct. The most valuable test-method, 

 however, is the production of endosmosis or exosmosis. If we take a cell with a soft and 

 thin wall, and add distilled water to it, it will imbibe a certain quantity of it and become 

 distended, and often the contents will become distinctly separated and visible within ; 

 whilst if a saturated solution of some salt, as chloride of calcium, be added, it will become 

 wrinkled and collapsed. On treating a solid or homogeneous body with water, it remains 

 unaltered, or perhaps swells slightly ; but on treating it with the solution of chloride of 

 calcium, no wrinkling or contraction occurs, and its appearance is unchanged. If the 

 outer coat be firm and resisting, the chloride will not cause it to contract and wrinkle. 



If there be two coats, the outer being firmer than the inner, the latter will be wrinkled 

 and collapsed, while the former retains its shape ; this is the ordinary occurrence in young 

 vegetable 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 comes into contact with 

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

 solution, 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 

 invisible, and thus apparently dissolves them ; sometimes also it really dissolves them. 

 Moreover 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 fre- 

 quently seen in the Desmidiacece " 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 structures, 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 allow- 

 ing the examination of the margins of the fragments. 



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

 refractive 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 



