42 TEXT-BOOK OF BACTERIOLOGY. 



On the contrary, ifc is our principle (and this deserves special 

 attention) to stain with very weak solutions, and allow a longer 

 time for their action. The peculiar differentiating- qualities of 

 the coloring matters then show themselves to the best advantage, 

 enabling us to perceive differences which would be entirely buried 

 under the quantity of color given by strong solutions. 



The best way of preparing the diluted solutions is by taking- 

 a glass bottle of about 10 cm. in height, which has a pipette in its 

 perforated cork, filling it two-thirds full of tap-water and slowly 

 adding as much of the saturated solution as leaves the liquid in the 

 thick part of the neck of the bottle just transparent. A little less 

 is better than too much. The same result may be obtained by 

 taking a saucer of tap-water and dropping- the concentrated so- 

 lution into it until the liquid begins to lose its transparency. 



It is true, solutions thus prepared have the bad quality of de- 

 composing after a time. The coloring matter is deposited more 

 and more, and the solution soon begins to lose its utility. It is 

 therefore well, for delicate staining operations, to prepare the 

 solutions anew every two or three weeks, or even oftener. But be- 

 fore commencing to stain it is well to consider whether we can em- 

 ploy indifferently all the stains hitherto mentioned, or whether cer- 

 tain colors are preferable for certain purposes. In fact, they have 

 their special properties. 



Gentian-violet is, as already mentioned, a very intense stain. 

 But this intensity is apt to become a disadvantage, for too long an 

 exposure overstains the object, the distinctions of texture are lost, 

 the whole preparation becomes indistinct and worthless for exam- 

 ination. Cautiously applied, however, gentian-violet is a partic- 

 ularly useful stain, and the more so as its effects are extremely en- 

 during and do not fade. 



Methyl-violet colors less intensely than gentian-violet, and is not 

 so apt to overstain an object, but it is less durable. 



Methyl-blue has far less coloring- power than the above-men- 

 tioned dyes. It requires a very long time to prod uce a perfect 

 staining, seldom overcharges with color, and yields tolerably durable 

 preparations. 



Fuchsin is a very beautiful, strong, lively color, which is par- 

 ticularly pleasing to the eye. It is net very apt to overstain and 

 gives preparations a high degree of durability, so that it is, in many 

 respects, preferable to the others. 



Bismarck-brown stains slowly and is no great aid to discrimi- 

 nation; therefore, it would probably not have remained long in use 

 had it not formerly been indispensable for the purposes of micro- 

 photography. 



