1889.] MICEOSCOPICAL JOUKNAL. 99 



This much it is only fair to say for these most minute but much 

 dreaded beings — taking them all in all, they do far more good than harm. 

 It is only a few that can enter the animal body, feed upon its elements, 

 and produce disease. On these I would make one or two remarks. 



Bacteria that grow apart from free air, in nitrogenous material, hav- 

 ing an alkaline reaction, can in some cases attack the animal system and 

 live on it. They seem to do this mainly by virtue of two of their 

 products : First, a poisonous vegetable alkaloid ; and, second, a solu- 

 ble ferment or solvent of the material making up the animal tissues. 



The primary attack comes from the poisonous alkaloid, which lowers 

 or destroys the vitality of the cells of the lymph, blood, or tissues, so 

 that they can no longer resist the chemical action of the ferment. The 

 second assault is made by the ferment in dissolving the animal cells and 

 tissues, and rendering them up in this condition as food for the living 

 bacterium. 



It should be noted here that the living corpuscles of the blood and the 

 living cells of the animal tissues are also endowed with power to pro- 

 duce a ferment or digesting principle, which is also deodorizing and 

 disinfectant, and which can safely dispose of any very small amount 

 of bacteria poison introduced. When the virus is introduced there is 

 at once set up a contest between the bacteria and their products on the 

 one hand, and the animal cells and their products on the other, and, as 

 in other contests, the stronger or the most advantageously situated pi*e- 

 vails. 



But the animal cells can acquire a power of resistance to alkaloid and 

 other poisons, and thereafter resist any moderate dose. Thus many of 

 you have acquired a comparative immunity from the poisonous action 

 of nicotine, the deadly alkaloid of tobacco ; and other men have acquired 

 immunity as regards alcohol, opium, ether, chloral, arsenic, or other 

 poison. This immunity is acquired by accustoming the animal cells to 

 bear the poison in question, and this is the explanation of the fact that 

 many contagious diseases will not readily occur a second time in the 

 same individual. 



As regards the reproduction of bacteria, many of them can double 

 their numbers every hour when placed in the best conditions for their 

 activity. In such circumstances, then, a single bacterium would in 

 twenty -four hours produce no less than 16,777,220. At the end of 

 forty-eight hours the offspring would amount to 281,^00,000,000, and 

 would fill a half-pint measure — all produced in two days from a single 

 germ measuring y-g-g-oo" °f an mcn m length. The figures are easily 

 tested. It is the old story of selling the horse at a cent for the first nail 

 in his shoe, two cents for the second, and so on, doubling the value each 

 time to the thirty-second nail. Whoever has worked out this problem 

 will not be surprised at this marvellous increase in the bacteria. 



Fortunately, however, bacteria can rarely so propagate themselves ; 

 they meet with all sorts of drawbacks, and thus, in spite of their enor- 

 mous fertility, the survivors are, in a general way, only enough to keep 

 up a fair balance in nature. The disease-producing bacteria, however, 

 have no such claim upon our forbearance, and in these the enormous 

 fecundity is a fact that we cannot too closely contemplate. Some, like 

 the bacillus of tuberculosis and glanders, propagate themselves slowly ; 

 but the great majority of the bacteria causing animal plagues will, in 



