MODES OF OKIGIN COMPARED 53 



may now be placed on the cover-glass just below one of the 

 minute air bubbles, and the space between it and the bubble then 

 carefully examined with the microscope. If the fluid in this region 

 is found to be free from particles, that will be the region to be 

 subsequently watched, and the edge of the air bubble will give the 

 necessary guidance for bringing the film of fluid into focus. 



The slip thus prepared may now be placed in an incubator, the 

 temperature of which is maintained at about blood-heat ; and after 

 three or four hours we may begin to make observations, replacing 

 the slip in the incubator after each observation. The time 

 necessary for the appearance of Bacteria will vary according to 

 the nature and strength of the infusion used ; and of course will 

 be more and more prolonged if lower temperatures are employed.' 



But if, in a motionless film of fluid, multitudes of living particles 

 subsequently appear, which are themselves almost motionless, how 

 can we account for their origin ? Three hypotheses present them- 

 selves. It may be said (a) that they have arisen through the 

 reproductive multiplication of one or more germs or organisms in 

 the film of fluid which, though visible, had escaped observation. 

 The difficulties standing in the way of our acceptance of this 

 explanation are these. The film is motionless, and also those first 

 appearing particles which gradually come into view in portions of 

 it where no such particles had been previously visible. No 

 multiplication by fission or other means can actually be observed 

 to take place among the first appearing particles in question, 

 though this ought to be easily observable if it really occurred at 

 the rate postulated. And lastly, if the subsequent large numbers 

 are to be accounted for by the occurrence of a reproductive 

 process taldng place among a few visible but unobserved germs, 

 these products of fission, being at first motionless, ought to be 

 aggregated here and there only, while as a matter of fact, this is 

 not the case — the distribution of the particles being more uniform. 



These various difficulties appearing fatal to this explanation of 

 the mode of origin of the multitudes of plastide-particles and 

 Bacteria, we are left with only two other possible modes of 

 origin : — either (6) they have been developed from a multitude of 

 diffusely disseminated invisible germs, or (c) they have been pro- 

 duced de novo in the fluid by a process of Archebiosis. 



' Thus I have found that with a temperature as low as 85° F., and the film 

 hermetically sealed, as in this mode of observation, the Bacteria usually begin 

 to appear somewhere between the twelfth and the eighteenth hour. 



