TEXT-BOOK OF BACTERIOLOGY. 41 



There are physical processes, depending on the laws of imbibi- 

 tion and diffusion, and on those peculiar phenomena which we call 

 surface attraction. But without doubt the principal part in the 

 development of the coloring and its specific forms must be assigned 

 to processes of a purely chemical nature. Although there may not 

 be such relations as can be expressed by formulas and give origin 

 to definite new combinations, yet the fine differences which are ob- 

 served in employing the various staining matters, their greater or 

 less coloring power, and in particular the marked preference of 

 special kinds of bacteria for special colors, certainly depend on 

 special chemical affinities, on a certain relationship between cer- 

 tain stains yielding and certain other colors absorbing substances. 



We liave, therefore, a right to regard the staining as the ex- 

 pression of a micro-chemical reaction, and if it is not always such 

 in a strict sense, it has the same value for us, since it enables us to 

 distinguish from each other by special criteria substances difficult 

 to separate. 



In order to be used they must be dissolved. Now, the anilin 

 colors, as also carmine and hasmatoxylon, dissolve equally well in 

 alcohol and in water. We use the first principally and make a sat- 

 urated alcoholic solution of the anilin colors, by putting into a bot- 

 tle containing alcohol as much of the drj' pigment as the alcohol 

 can dissolve. It is better to put in a still greater quantity, shaking 

 it up well, filtering it after a few days, and using the concentrated 

 solution thus obtained. One point, however, requires particular at-' 

 tention. Anilin colors are seldom if ever sold in complete purity. 

 They are generally mixed with other substances, particularly dex- 

 trin and soda, not as an intentional adulteration, but only to meet 

 certain technical requirements. 



These admixtures often dissolve slowly and imperfectly in the 

 alcohol, and remain at the bottom of the bottle as a thick sediment, 

 which, when we employ the solution, may cause deposits and other 

 unpleasant consequences. 



The methyl-blue especially often contains quantities of foreign 

 substance, and is therefore sometimes dissolved in distilled water 

 instead of in alcohol. 



Bismarck-brown, too, keeps better in an aqueous solution, or in 

 a mixture of equal parts of glycerin and water, than in alcohol. 



The concentrated aqueous or alcoholic solutions are the source 

 from which we take the staining matter we require, and which is 

 kept on hand for that purpose. An attempt to use these solu- 

 tions in their concentrated form would produce effects greatly too 

 rapid and too intense. They overstain the preparation. 



