772 THE POPULAR SCIENCE MONTHLY. 



the bodies of those animals which had been inoculated with a very 

 minute quantity of blood. The symptoms are very characteristic, and 

 the disease at one time caused an enormous mortality among cattle in 

 France. By the opponents of the bacterium hypothesis it might, of 

 course, be urged that in the inoculation experiments other morbid ma- 

 terials were simultaneously conveyed, and that the transmission of the 

 disease was due to their presence. To meet this objection, and to fulfill 

 the second condition laid down in the last paragraph, experiments for 

 cultivating the organism were set on foot in the following manner : 

 A drop of blood taken from an animal that had died from anthrax was 

 put into a glass flask containing an infusion of yeast, which had been 

 carefully treated and proved to be free from organisms. In twenty- 

 four hours the liquid, previously clear, was seen to be full of very 

 light flakes, which, when examined under the microscope, were found 

 to be masses of organisms resembling those contained in the blood. A 

 drop taken from this first flask was added to a second and produced 

 the same effect, and a drop from this was added to a third, and so on 

 till a tenth flask was thus charged with organisms. In this way the 

 organism was enormously multiplied and completely freed from the 

 admixture of any other substance. Yet when a drop was taken from 

 the twentieth or even the fiftieth flask of such a series and inserted 

 under the skin of a sheep it caused anthrax or splenic fever, attended 

 by the same symptoms as those produced by the drop of blood taken 

 from the first animal. It is impossible to conceive of any clearer proof 

 that the organism is the sole cause of the disease. So crucial a test, 

 however, can not be applied in every case, for many of the infectious 

 diseases w r hich are the scourge of mankind do not affect the lower 

 animals, and it is therefore impossible to make trial of the organisms 

 found in connection therewith. Besides anthrax, there are other in- 

 fective diseases in animals which have been proved to be due to bac- 

 teria, and these facts strongly support the belief that the infectious 

 diseases of mankind are due to the invasion of similar organisms. It 

 is, however, impossible as yet to dogmatize upon this subject. There 

 have already been too many assertions and inferences drawn there- 

 from which have turned out to be unwarranted. It is comparatively 

 easy for skilled observers to detect the presence of micro-organisms, 

 and, whenever uniformity of appearance is demonstrable in connection 

 with a given disease, a decided addition has been made to our knowl- 

 edge. For reasons above given, the next point, viz., the determining 

 whether the organisms are the cause of the disease, is surrounded with 

 great difficulties. The discoveries, however, with regard to splenic 

 fever strongly support the view that bacteria are the efficient agents 

 of contagious diseases. 



Space will not permit me to do more than allude to the various 

 theories that have been advanced with regard to the manner in which 

 these tiny organisms produce disease. It was at first thought that they 



