94 TOBACCO LEAF. 



ever, and that most frequently useful for their determi- 

 nation, is that the products they form are distinctly dif- 

 ferent. Some liberate gas, and the gases from various 

 species differ in composition. In other cases, substances 

 of pronounced odor or flavor are developed, as in the pu- 

 trefactive fermentations, and in those of ripening cheese 

 and ripening cream. The disease germs accomplish 

 their fatal results, it is now believed, more frequently 

 through the poisons they form in the blood poisons 

 similar, chemically, to the active principles of snake 

 venom than through any direct action of their own. 

 Ordinarily, the conditions favorable to the develop- 

 ment of one species of bacterium are also such as permit 

 the development of other species. Hence, under natural 

 conditions, a single species rarely occurs alone. By se- 

 lection of the most congenial nutritive medium for a 

 given species of which it is desired to secure a pure cul- 

 ture, that is, a colony in which no foreign species ex- 

 ists, and by regulation of temperature so that that 

 most favorable to the species in question may be main- 

 tained, it is possible to gradually eliminate undesirable 

 species from a series of cultures, and secure a culture in 

 which only the species desired remains. The process is 

 much hastened, first, by using a sterilized culture me- 

 dium and sterilized apparatus ; second, by preventing 

 access of foreign germs from the air this is accom- 

 plished by filtering the air to which the solution is ex- 

 posed, through cotton- wool, or some similar substance, 

 which removes all floating dust from the air, including 

 the dried germs ; third, by diluting the primary material 

 from which the germs are taken, and using a very small 

 quantity of the diluted substance to act as a starter for 

 the new solutions ; often, this process introduces into 

 some of the cultures very few, if any, foreign species, so 

 that these cultures may be made the basis of further op- 

 erations, and others, less pure, be rejected at once. 



