STAINING REAGENTS. 31 



pound granular corpuscles, caseine, slender filaments of fibrin (Cohn), 

 or fibrin becoming granular, may in turn be distinguished from the 

 more resistant micrococci. In tissues it is a good rule, if no other 

 chemicals or staining reagents are at hand, and where the presence of 

 masses of micrococci is suspected, to macerate the sections for an 

 hour in a ten per cent, solution of liquor potassas, and then stain 

 with a solution of iodine ; by this method the bacteria are stained 

 brown, whilst fat granules, &c. remain uncoloured. 



Staining Reagents. 



24. It is at once evident that, by careful attention to details such 

 as the above, micro-organisms can be distinguished with tolerable 

 certainty ; but it is only by calling to our aid staining methods that 

 we are able to study the minute .structural peculiarities of these 

 organisms ; in fact some of the most important peculiarities yet 

 recognised come out only as chemical colour reactions. Staining 

 methods, as applicable to the study of micro-organisms, are daily 

 becoming more perfect, and though in some cases they are being 

 elaborated, the general tendency is undoubtedly towards a simplifi- 

 cation of the processes. To Weigert and Koch we are especially 

 indebted for most of the elegant methods now at our command, 

 — methods which are based on the use of aniline colours. 



It may be laid down as a general statement that micro-organisms 

 react to staining fluids very much as do the nuclei of cells. This 

 holds good so far, that most nucleus tinting materials will impart a 

 similar tint to micro-organisms; beyond this, however, it is found that 

 the micro-organisms hold the colouring matter more tenaciously than 

 do the cell-nuclei, and that the nuclei may be decolorised by the 

 use of certain reagents, such as carbonate of potash, recommended 

 by Koch, acetic acid, or one of the mineral acids, nitric or 

 hydrochloric. 



In this connection it must be remembered that many micro-organ- 

 isms can at present only be classified according to their reactions 

 with the colouring and some one of the various decolorising reagents. 

 Some take on one stain more readily and retain it more persistently 

 than any other, and some, which in the first instance take up the 



