THE USE OF STRONGER STAINING SOLUTIONS. 113 



1"5 grm. of methylene blue may be used. Both 

 these solutions may be kept for an unlimited period, 

 and on this account are exceedingly useful, for, 

 although, not quite so powerful as the ones prepared 

 with aniline water, yet they are quite suitable for 

 occasional use^ and are always at hand. 



Now, when fluorescent bacteria are treated with one 

 of these solutions, it will be seen that even those 

 which, only absorbed the aqueous solutions in a very 

 imperfect manner are readily and intensely stained. 

 Of course the others which were affected by the 

 aqueous solutions are also stained by the stronger ones. 

 We shall, however, be able to observe many important 

 differences in the behaviour of the various species 

 when we proceed to the use of aqueous alcohol for 

 decolourising. 



Most of those forms which are only stained with 

 difficulty, retain the colouring matter more firmly 

 than those which are more easily stained; they are 

 able, in consequence, to resist the decolourising action 

 of very dilute alcohol. Indeed, many kinds are not 

 decolourised at all with alcohol.' To this rule there 

 are, however, a few exceptions; the typhoid bacillus, 

 for example, which is only stained with considerable 

 difficulty, can be very readily decolourised. 



' This is, of course, only relative. All bacteria ai'e de- 

 colourised in time with alcohol, as with, water, only with 

 some species the operdftion may take days, or even weeks. 



I 



