160 BACTERIOLOGY. 



so that it is better to prepare it fresh in small quantities 

 when needed than to employ old solutions. Solutions 

 older than fourteen days should not be used. 



The three solutions just given may be used for cover- 

 glass preparations in the ordinary way. 



In some manipulations it becomes necessary to stain 

 the bacteria very intensely, so that they may retain 

 their color when exposed to the action of decolorizing 

 agents. These methods are usually employed when it 

 is desirable to deprive surrounding objects or tissues of 

 their color, in order that the stained bacteria may stand 

 out in greater contrast. It is in these cases that the 

 staining-solution with which the bacteria are being 

 treated is to be warmed, and in some cases boiled, so as 

 further to increase its penetrating action. When so 

 treated, certain of the bacteria will retain their color, 

 even when exposed to very strong decolorizers. The 

 tubercle bacillus is distinguished from the great ma- 

 jority of other bacteria by the tenacity with which it 

 retains the color when treated in this way ; it is an 

 organism difficult to stain, but when once stained is 

 equally difficult to rob of its color. 



DECOLORIZING-SOLTJTIONS. As regards the employ- 

 ment of decolorizing-agents, it must always be borne in 

 mind that objects which are easily stained are also easily 

 decolorized, and those that can be made to take up the 

 staining-material only with difficulty are also very diffi- 

 cult to rob of their color. The most common decolor- 

 izer in use is probably alcohol not absolute alcohol, 

 but alcohol containing more or less of water. Water 

 alone has this property, but in a much less degree than 

 dilute alcohol. On the other hand, a much more ener- 

 getic decolorization than that possessed by either alone 



