TEXT-BOOK OF BACTERIOLOGY. 47 



Acetic acid, as Weigert has taught, is almost indispensably nec- 

 essary for producing 1 a decided nucleus staining-. This acid has the 

 property of making- the protoplasm swell, while it causes a contrac- 

 tion of the nuclei. At the same time, it removes the coloring- mat- 

 ter from the former, but fails to penetrate into the latter, which 

 therefore appear more prominently than their pale surroundings, 

 and particularly attract the eye by their staining 1 . 



We usually add three or four drops of acetic acid to about 20 

 cm. of water, and wash the sections in this solution for a consider- 

 able time. 



The other acids act much more energetically, and some of them 

 in a different manner also. These are muriatic acid, sulphuric acid, 

 and nitric acid, of which the last mentioned is a particularly strong 

 bleaching agent. None of them should be used without caution, as 

 they exercise a destroying influence on most dyes and are capable 

 of neutralizing the fastest stains. 



There is more power in alcohol than in water, and therefore it 

 is preferable in the case of strong stains. As may be imagined, too, 

 the acidulated alcohol surpasses the acidulated water in its decolor- 

 ing power. All the decoloring agents may therefore be set down 

 in a list of increasing strength with water at the bottom and nitric- 

 acid alcohol at the top. 



Iodine has a somewhat similar action to those just mentioned, 

 though its application is not the same. Iodine is employed with 

 arsenite of iodide of potassium (iodine 1 part, arsenite of iodide of 

 potassium 2 parts, water 300 parts). This mixture exercises a very 

 peculiar influence on preparations which have been treated with 

 anilin water and gentian-violet. It forms a deposit with the color- 

 ing matter, which, however, only adheres to the bacteria, and can 

 be washed out of all other parts of the tissue, the nuclei included. 



We thus obtain an isolated bacterial staining which is particu- 

 larly suited for special purposes. Such a process is that recom- 

 mended by Gram. 



Remember that we possess stains, such as carmine and picro- 

 carmine, which particularly affect the nuclei, and it may readily be 

 imagined that by the union of the isolated bacteria, staining on the 

 one hand and the isolated nucleus coloring on the other hand, we 

 can obtain the most perfect distinction of all the separate compo- 

 nent parts of such a preparation. If we have stained the bacteria 

 alone according to Gram's method, we let the second stain act upon 

 the nuclei, and thus w r e get a contrast in one picture which shows 

 the mutual relations of bacteria and tissue in the most satisfactory 

 manner. 



