17 



tention by Pasteur, as well as to the facility of its manip- 

 ulation, has secured an unfortunately extensive and per- 

 sistent employment. The defects become apparent when 

 we consider the vital properties of bacteria. Different 

 species require different pabulum and various tempera- 

 tures for their successful cultivation. The bacillus sub- 

 tilis (Cohn), for example, grows luxuriantly in a simple 

 infusion of hay, which is ordinarily slightly acid in reac- 

 tion ; the bacillus anthracis, which is morphologically 

 similar, indeed almost identical with the former, grows 

 very slowly or not at all in the same infusion ; the addi- 

 tion of a little magnesia or other base, sufficient to render 

 the liquid somewhat alkaline, reverses the relative repro- 

 ductive activity of the two. The hay bacillus (b. subtilis), 

 again, can reproduce at a temperature incompatible with 

 the reproduction of the b. anthracis. If, then, the two be 

 sown in the same sterilized hay infusion, the crop will be 

 determined largely by the reaction and by the tempera- 

 ture of the liquid. Another feature to be remembered is 

 the variable rapidity with which different species multiply, 

 even under the most favorable surroundings ; for as 

 Nageli has shown, if two bacterial varieties, A and B, be 

 present in a liquid adapted to each, A dividing its cell 

 into two in twenty-five minutes, B in forty minutes, the 

 latter, even if present at first in 1,800,000 times the 

 number of the former, will in eighty hours have been 

 stifled by A. In a mixed cultivation in other words 

 the quickest to propagate will, cceteris paribus, in a 

 few hours or days remain alone a principle with whose 

 applicability to higher organisms we are of course famil- 

 iar. It is evident, then, that the best method of iso- 

 lation is that which affords (first) the greatest security 

 against the intrusion of other organisms than that under 

 cultivation, and since such intrusion cannot by any 

 method at present employed be with certainty prevented 

 (second), the greatest probability of the detection of such 

 invasion by other bacteria. The isolation cultures in 

 flasks suffer the same dangers of adulteration as other 

 methods, aggravated somewhat by the increased difficulty 



