TEXT-BOOK OF BACTERIOLOGY. 51 



If Gram's method is to be employed, the cover-glasses are stained 

 ■with a hot, strong, saturated anilin-water gentian-violet solution 

 for one or two minutes ; they are next put immediately into iodide 

 of potassium solution for about half a minute, and lastly washed 

 with alcohol till no more stain can be removed. 



They may then be examined at once, or thej'^ may be completed 

 with a contrast stain such as saflranine, carmine, or a very weak 

 solution of Bismarck-brown. An alcoholic solution of eosin is par- 

 ticularly recommended, since it displays the cellular portions of the 

 blood with rare clearness. The superfluous eosin is removed with 

 distilled water or dried with blotting-paper and mounted in balsam. 



The immediate examination of the stained cover-glass prepara- 

 tions in water is particularly recommended in most cases, because 

 it injures the micro-organisms least and shows them to us in a 

 condition more nearly approaching their natural form. It is true, 

 the hollow slide is much superior for this purpose, and that the al' 

 cohol, the staining fluid, the iodide of potassium, the acids— in short, 

 the whole process of preparation, affects the appearance of the bac- 

 teria, and leaves us only mummies and corpses to examine. 



The protoplasmic body of the cell contracts, the membrane alters 

 its appearance, granulations are produced which are quite foreign 

 to the living organism. 



But in spite of all this, we can on no account renounce the stain- 

 ing system, which has its advantage and is quite indispensable for 

 our investigations. 



In the preparations mounted in water the cells are still ex- 

 panded and full of juices, the membrane comes out clearly, the sep- 

 arate micro-organisms show body and mass. 



All this is changed in a most strikmg manner as soon as we 

 preserve such an object, whenever it is prepared for comparative 

 investigations or for lecture-room demonstrations. In this case 

 the object, after having been thoroughly air-dried, has to be mounted 

 in Canada balsam. 



This medium causes the bacteria to shrink considerably. They 

 take a poor, shrivelled appearance, and when re-examined in Canada 

 balsam, what had before been seen in water will frequently be rec- 

 ognized as the same with difHcultj'. 



Therefore permanent preparations should only be made when 

 necessary for the purposes above mentioned, or when there is some 

 • other particular reason for wishing to preserve an object. 



Two special applications of the cover-glass staining for particu. 

 lar purposes will be most appropriately mentioned here. The en- 

 dogenous spores of the bacteria are distinguished by a firm cover- 



