TEXT-BOOK OF BACTERIOLOGY. 35 



t'd^-es a brisk evaporation is constantly taking- place and causing- 

 all sorts of currents in the liquid, so that the whole preparation is 

 in continual motion. As this motion is sometimes quick and irreg- 

 ular, it sweeps away the bacteria from under the eye of the obser- 

 ver; they are driven about in rapid currents, and it becomes im- 

 possible to decide one of the most important questions with regard 

 to many micro-organisms whether they possess the faculty of 

 voluntary movement. Besides this, the rapidity of the drift 

 movements is generally so great that it is extremely difficult to 

 satisfactorily observe the finer peculiarities of form in these wander- 

 ing bacteria. Lastly, too, a prolonged examination of such prepara- 

 tions is rendered impossible by the evaporation, which soon ends 

 in the complete drying up of the liquid. 



These are the very grave objections to this method of proced- 

 ure, and which prevent its general use. As a rule, it is only em- 

 ployed tentatively for instance, to see whether a fluid contains bac- 

 teria or not; further, also, when we wish to take a speedy glance, 

 in order to know whether we have bacilli or micrococci to deal 

 with, etc. In all cases, however, where more exactness is required, 

 this most simple of all proceedings is abandoned, since, fortunately, 

 means have been found to a.void these inconveniences. 



The end of a platinum wire is bent into a loop. Special atten- 

 tion is called to the manner of making such a loop. It is no mere 

 caprice that calls forth this advice. Experience will prove that this 

 simple tool is of great importance for many technical manipulations, 

 and particularly in the cultivation of bacteria it is almost con- 

 stantly in use, while the success of many experiments depends en- 

 tirely on the suitability of the loop employed. The great point is 

 to see that the loop forms a closed circle, so that it is able to raise 

 a full drop. It should, further, be smoothly rounded and neither 

 too large nor too small; the size of a capital O in Roman print 

 will be found to best answer general requirements. If such a loop 

 after heating- in a flame be plunged into the liquid, a drop will 

 be found hanging to it when it is withdrawn. 



Carefully and slowly touch the centre of a cover-glass with it, 

 the drop leaves the loop and passes to the glass, on which it quietly 

 remains. A successful "drop" should be as shallow as possible, 

 should have smooth, even edges, and be about the size of a pea at 

 the most. Now take a hollowed slide with a shallow cavity and 

 brush around the edge some soft vaselin, or some other air-tight 

 unguent, turn the slide over so that the hollow may be on the 

 under side, and press it down upon the cover-glass, which, of 

 course, sticks to the vaselin. Take up the slide with the cover- 



