Page:Popular Science Monthly Volume 30.djvu/796

772 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 materials 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 which 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 infective diseases in animals which have been proved to be due to bacteria, 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 therefrom 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 knowledge. 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