204 THE MICROSCOPE AND 4TS REVELATIONS. 



its blackening of fatty matters and the medullary substance of nerve-fibres. 

 And the Embryologist finds it of peculiar value in giving firmness and distinct- 

 ness to the delicate textures with which he has to deal. Various degrees of dilu- 

 tion of the 1 per cent solution will be needed for these different purposes. 

 Mr. Parker further states (loc. cit.) that he has found this agent very service- 

 able in the preparation of delicate Vegetable structures. " The acid seems to be 

 taken up by each granule of the protoplasm, and these to be decomposed, giv- 

 ing to the granule the characteristic gray color, thus at the same time both 

 hardening and staining." A mixture of 9 parts of a l-4th per cent, solution of 

 Chromic acid, with one part of a 1 per cent solution of Osmic acid, answers for 

 many purposes better than osmic acid alone, the brittleness produced by its use 

 being completely avoided. After being subjected to this agent, the specimens 

 should be treated with 30 per cent alcohol, gradually increased in strength to 

 absolute. 



200. Staining Processes. Much attention has been given of late 

 years to the use of agents, which, either by simply dyeing, or by chemi- 

 cally acting on Organic substances, in different modes and degrees, serve 

 to differentiate the different parts of organs or tissues of complex struc- 

 ture, and to render more distinct such delicate features in preparations 

 mounted in transparent media, as might otherwise escape notice. The 

 agents which merely dye the tissues are for the most part Coloring mat- 

 ters of Vegetable or Animal origin; those which act upon them chemically 

 are Mineral substances. The dyes need generally to be ' fixed ' by some 

 ' mordant;' but the effects of chemical agents are usually permanent. 

 The staining-processes maybe used either before or after section -cutting, 

 according to circumstances. Where the substance is in mass, and is not 

 readily penetrable by the staining fluid (which is especially liable to be 

 the case where it has been hardened in chromic acid), it is generally bet- 

 ter to stain the sections after cutting, if they hold sufficiently well 

 together to bear being transferred from one fluid to another. And if the 

 substance is to be imbedded in gum, and cut with the freezing Micro- 

 tome, it is generally preferable to stain the sections after they have been 

 cut; as the processes necessary for the removal of the gum would be likely 

 also to remove the dye. But where the substance to be cut has to be 

 penetrated by wax or paraffine, it is better that the staining should be 

 effected in the first instance. As a general rule, it is better that where 

 the substance is to be stained en masse, the staining fluid should be weak 

 and its action slow; because in that mode the stain is more equably dif- 

 fused. When, on the other hand, the process is made use of with thin 

 sections, it is convenient that the action should be more rapid, and the 

 staining fluid may therefore be stronger; but unless its operation be care- 

 fully watched so as to be stopped at the right stage, the whole tissue may 

 be deeply dyed, and the value of the selective staining altogether lost. 



201. It will generally be found convenient to carry-on the staining of 

 thin sections either in watch-glasses, or in small cups of white porcelain; 

 but care must be taken not to place many sections together so as to lie 

 one upon the other, as this will prevent the staining from being uniform. 

 Small delicate sections may often be advantageously stained upon the 



flass slides upon which they are to be mounted; a pool of the staining 

 uid being made upon the slide, to be removed, when the staining has 

 proceeded far enough, by the small glass Syringe ( 127). It is even 

 possible to stain a section after it has been covered with thin glass, by 

 depositing the fluid in contact with one edge of the glass cover, and 

 drawing it through by applying a bit of blotting-paper to the opposite 

 margin; and the process may thus be performed while the section is actu- 

 ally under observation on the stage of the Microscope, the staining liquid 



