556 VRA GMENTS OF SCIENCE 



microscope, warmed it suitably, and observed the subse- 

 quent 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 hundred 

 times the length of the original rods. The same filament, 

 in fact, was frequently observed to stretch through several 

 fields of the microscope. Sometimes they lay in straight 

 lines parallel to each other, in other cases they were bent, 

 twisted, and coiled into the most graceful figures; while 

 sometimes they formed knots of such bewildering com- 

 plexity 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 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 perplexity 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 inoculation 

 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 



