118 THE FLOATING-MATTEB OF THE AIE. 



The tray of tubes proved so helpful in enabling me 

 to realize mentally the distribution of germs in the air, 

 that on the 9th of November, 1875, 1 exposed a second 

 tray containing one hundred tubes filled with an infusion 

 of mutton. On the morning of the 1 1th six of the ten 

 nearest the stove had given way to putrefaction ; three 

 of the row most distant from the stove had yielded, 

 while here and there over the tray particidar tubes were 

 singled out and smitten by the infection. Of the whole 

 tray of one hundred tubes, twenty-seven were either 

 muddy or cloudy on the 11th. Thus, doubtless, in 

 an infective atmosphere, are individuals successively 

 struck down. On the 12th all the tubes had given 

 way, but the differences in their contents were extraor- 

 dinary. All of them contained Bacteria, some few, 

 others in swarms. In some tubes they were slow and 

 sickly in their motions, in some apparently dead, while 

 in others they darted about with rampant vigour. These 

 dififerences are to be referred to differences in the ger- 

 minal matter, for the same infusion was presented 

 everywhere to the air. Here also I imagine we have a 

 picture of what occurs during an epidemic, the difference 

 in number and energy of the Bacterial swarms resem- 

 bling the varying intensity of the disease. It becomes 

 obvious from these experiments that of two individuals 

 of the same popidation exposed to a contagious atmo- 

 sphere, the one may be severely, the other lightly 

 attacked, though, as regards susceptibility, the two 

 individuals may be as identical as two samples of one 

 and the same mutton-infusion. What I have already 

 said regarding the 'preparedness' of contagium has its 

 application here. 



The parallelism of these actions with the progress 

 of infectious disease may be traced still further. The 

 ' Times,' for example, of January 1 7, 1876, contained a 

 letter on typhoid fever, signed ' M.D.,' in which occurs 



