THE MORPHOLOGY OF BACTERIA 7 



which at the same time serves to spread the water. There should 

 be a faint, only barely perceptible cloudiness. Then with an 

 oese made of delicate platinum wire transfer an oese-full from 

 the first cover-glass to the second, and with a single circular 

 movement spread the drop as before into a thin layer. In the 

 same manner transfer from the second to the third cover-glass, 

 and so again to the fourth. On the third and fourth cover- 

 glasses the bacteria will be sufficiently scattered and few in 

 number to remain separated. The thin watery films dry quickly 

 in the air. They are then fixed, and this is one of the most im- 

 portant operations, since most manipulators overheat in fixing. 

 To do this, hold the two opposite edges of the cover-glass between 

 the thumb and forefinger, and pass once through the flame so 

 rapidly that the fingers feel no pain. The films are then ready 

 for the mordant, which in Lowit's method is prepared as follows : 

 Dissolve 5 g. of tannic acid in 20 c.c. of water, and filter twice ; 

 to 10 cc. of this add 5 cc. of a saturated solution of copper 

 sulphate, and i cc. of a saturated alcoholic solution of fuchsin. 

 Filter the mixture twice. The necessary quantity of the mor- 

 dant is then placed upon the films and allowed to act for 23 

 minutes in the cold. The films then are very thoroughly washed 

 in water, and are ready for staining. The stain is the ordinary 

 analine water gentian violet or fuchsin. The staining is done 

 cold for 3-5 minutes. The films are then thoroughly washed 

 in water, or for a few seconds in 50 per cent alcohol. They are 

 then mounted and examined as usual. One important point is 

 that all the solutions should be fresh and carefully filtered. 

 This applies as well to the tannic acid solution. The copper 

 sulphate solution is of course durable. 



2. THE FORMS OF BACTERIA 



Bacteria present a great variety of forms. There are, how- 

 ever, certain morphologic types to which the majority of them 

 conform. The most common of these are cocci or spherical bac- 



