268 THE FLOATING-MATTER OF THE AIR. 



possible for the eye to trace the individual filaments 

 through the confusion. 



Had the observation ended here an interesting 

 scientific fact would have been added to our previous 

 store, but the addition would have been of little practical 

 value. Koch, however, continued to watch the filaments, 

 and after a time noticed little dots appearing within 

 them. These dots became more and more distinct, until 

 finally the whole length of the organism was studded 

 with minute ovoid bodies, which lay within the outer 

 integument like peas within their shell. By-and-bye 

 the integument fell to pieces, the place of the organisms 

 being taken by a long row of seeds or spores. These 

 observations, which were confirmed in all respects by 

 the celebrated naturalist, Cohn of Breslau, are of the 

 highest importance. They clear up the existing per- 

 plexity regarding the latent and visible contagia of 

 splenic fever ; for in the most conclusive manner, Koch 

 proved the spores, as distinguished from the rods, to 

 constitute the contagium of the fever in its most deadly 

 and persistent form. 



How did he reach this important result ? Mark the 

 answer. There was but one way open to him to test 

 the activity of the contagium, and that was the inocu- 

 lation with it of living animals. He operated upon 

 guinea-pigs and rabbits, but the vast majority of his 

 experiments were made upon mice. Inoculating them 

 with the fresh blood of an animal suffering from splenic 

 fever, they invariably died of the same disease within 

 twenty or thirty hours after inoculation. He then 

 sought to determine how the contagium maintained its 

 vitality. Drying the infectious blood containing the 

 rod-like organisms, in which, however, the spores were 

 not developed, he found the contagium to be that which 

 Dr. Sanderson calls ' fugitive.' It maintained its power 



