270 XXI. BACTERIA YEAST. 



by fingers or forceps, each being covered with a very thin film of 

 the material. The cover- glasses should remain in a chamber free 

 of dust until they are completely air-dry. 



Bacteria show themselves to be very resistant to water, alcohol, 

 ether, dilute mineral acids, acetic acid and weak alkalies, and we 

 can proceed to test them with these reagents. We make use of 

 50 per cent, acetic acid, or 12 per cent, sulphuric acid, or, better 

 still, 3 per cent, potash. In this last the preparation becomes at 

 once as transparent as necessary, the bacteria usually standing out 

 quite sharply ; and, by swelling, their bulk is so far enlarged that 

 a less strong magnification is made useable. Since large masses 

 of oil, should they be in the preparation, injure observation very 

 greatly, we should take care to remove them. This can be effected 

 either by warming the dry preparation, covered with a drop of 

 potash over a spirit flame, until bubbles begin to be formed, by 

 which means the fat is saponified ; or the dry preparation can be 

 treated for a few minutes in a watch-glass with chloroform, and 

 then with absolute alcohol, and after the evaporation of this latter 

 the potash applied. 



An exception to this capacity for resistance possessed by 

 bacteria is afforded, amongst others, by Spirillum (Spirochcete) 

 Obermeieri, the spirillum of intermittent fever, and some other 

 Spirillum, which are destroyed by these reagents. We can, 

 however, in general decide that regularly-formed bodies, which 

 resist the action of alcohol and ether, the acetic acid and potash 

 tests (even with warming, as above), can be considered as bac- 

 teria ; although in certain cases deception by specially resistant 

 and regular granulations is not entirely excluded from the range 

 of possibilities. 



Staining Methods. Their staining properties are also of value 

 in the determination of bacteria. It is true that other minute 

 bacteria-like bodies can take up stains, and some bacteria will not 

 stain at once, so that here also in some circumstances caution is 

 necessary. Specially used are the so-called basic aniline colours, 

 and above all methyl violet, gentian violet, methylene blue, 

 fuchsin, Bismarck brown and Vesuvin, are used for the stain- 

 ing of bacteria. The bacteria not only take up these stains eagerly, 

 but retain them very energetically, far more energetically than 

 they are at the same time retained by the neighbouring tissue 

 elements in the preparation, if the bacteria are stained in situ. 

 These stains are used in saturated watery solutions, which must 



