i 5 o THE POPULAR SCIENCE MONTHLY. 



during his spare time, various original and ingenious devices to the 

 investigation of splenic fever. He studied the habits of the rod-like 

 organisms, and found the aqueous humor of an ox's eye to be particu- 

 larly suitable for their nutrition. With a drop of the aqueous humor 

 he mixed the tiniest speck of a liquid containing the rods, placed the 

 drop under his microscope, warmed it suitably, and observed the sub- 

 sequent action. During the first two hours hardly any change was 

 noticeable ; but at the end of this time the rods began to lengthen, 

 and the action was so rapid that at the end of three or four hours they 

 attained from ten to twenty times their original length. At the end of 

 a few additional hours they had formed filaments in many cases a hun- 

 dred times the length of the original rods. The same filament, in fact, 

 was frequently observed to stretch through several fields of the mi- 

 croscope. Sometimes they lay in straight lines parallel to each other; 

 in other cases they were bent, twisted,' and coiled, into the most grace- 

 ful figures ; while sometimes they formed knots of such bewildering 

 complexity that it was impossible 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- 

 by the integument fell to pieces, the place of the organism 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 first 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 distin- 

 guished 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 inoculation with it of living animals. He operated 

 upon Guinea-pigs and rabbits, but the vast majority of his experiments 

 were made with 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. Dry- 

 ing 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 of infection for five weeks at the farthest. He then dried- 

 blood containing the fully-developed spores, and exposed the sub- 



