454 



THE POPULAR SCIENCE MONTHLY. 



must lead to a scientific determination of the existence and nature of 

 the bacteria-germs. His beautiful experiments on the decomposition 

 of vapors, and the formation of actinic clouds by light, led him to 

 experiment on the floating matter of the air, and with what results is 

 widely known. Confined and undisturbed air, however heavily charged 

 with motes, becomes at length, by their deposition, absolutely clear, 

 so that the path of the electric beam is invisible across it. From this, 

 and associated indications, he acutely inferred that " the power of 

 developing life by the air, and its power of scattering light, would be 

 found to go hand in hand ; " so that a beam of light sent across the 

 air into which infusions might be placed and examined by the eye, 

 rendered sensitive by darkness, might be utilized with the best results 

 in determining the existence of bacteria-germs. To bring the idea to 



':..-''' ''. ', 





Fig. 6. 



a practical result a number of chambers were constructed with glass 

 fronts. At two opposite sides facing each other a couple of panes of 

 glass were placed to serve as windows, through which the electric 

 beam might pass. A small door was placed behind, and an ingenious 

 device was arranged to enable a germ-tight pipette to have free lat- 

 eral, as well as vertical, motion. Connection with the outer air was 

 preserved by means of two narrow tubes inserted air-tight into the 

 top of the chamber. The tubes were bent several times up and down, 

 "so as to intercept and retain the particles carried by such feeble 

 currents as changes of temperature might cause to set in between the 

 outer and the inner air." 



Into the bottom of the boxes were fitted large air-tight test-tubes, 

 intended to contain the liquid to be exposed to the action of the mote- 

 less air. 



" On September 10th the first case of this kind was closed. The 

 passage of a concentrated beam across it showed the air within it to 

 be laden with floating matter. On the 13th it was again examined. 

 Before the beam entered, and after it quitted the case, its track was 

 vivid in the air, but within the case it vanished. Three days quite 



