6 TEE POPULAR SCIENCE MONTHLY 



which we call cocci, and the rod-like forms or bacilli. These early 

 studies were almost exclusively botanical in nature and it was not until 

 1872 that Cohn could include definite disease-producing bacteria in his 

 classification of the vegetable microorganisms. 



Bacilli had been found, it is true, as early as 1850 in diseased ani- 

 mals, for example, the anthrax bacillus in animals dying of splenic 

 fever. So also Schonlein in 1839 had discovered a vegetable parasite, a 

 mycelial form, higher than the bacteria, in the disease of the skin 

 known as favus; Malmsten in 1848 had found a somewhat similar form 

 in barber's itch, and Bassi about 1832 had demonstrated that a disease 

 of the silkworm was due to a minute cryptogamic plant. But the im- 

 portance of these observations was not widely appreciated and no gen- 

 eral relation was established between bacteria and disease in man. 



Likewise, theories of infection which explained disease as due to 

 invisible microorganisms had been propounded as early as 1762, as for 

 example that of Plenciz, which, based on Leeuwenhoek's discoveries, 

 ascribed to every disease its particular microorganism, explained the 

 decomposition of animal and vegetable material as due to microorgan- 

 isms, postulated the growth of bacteria in living tissues and suggested 

 the possibility of the transmission of disease virus by the air. Such 

 views, naturally, were without experimental basis and without even an 

 objective knowledge of the microorganisms supposed to be etiologically 

 concerned. In other words the propounder of this theory, as others 

 after him, believed more than he could prove. By the middle of the 

 century, however, observations on bacteria, largely as the result of the 

 labors of botanists, were accumulating, and views about spontaneous 

 generation, fermentation and infection were being discussed, but the 

 fundamental experiments necessary to settle these problems were yet 

 to be made, and, curiously enough, it was to a chemist, influenced by 

 the methods of physics, who was to establish bacteriology as a biological 

 science and to give to it the important place in medicine which it has 

 occupied for the past thirty years. 



Pasteur was this chemist, and his first great discovery was in crys- 

 tallography, the explanation of the behavior of one of the tartaric acids 

 to polarized light. This acid obtained from the lees of wine was, 

 unlike other acids of the group, inactive to polarized light. This 

 inactivity Pasteur demonstrated to be due to the fact that it was 

 made up of two isomeric constituents. The crystals of one of these 

 constituents bore hemihedral facets on the right side and rotated the 

 plane of polarized light to the right, and those of the other bore 

 similar facets on the left, and therefore, rotated to the left, but, as 

 Pasteur found, when combined, these crystals did not rotate the plane 

 of polarized light at all. This, the first of his discoveries, was in 1818, 

 the year that Virchow was investigating typhus fever in Silesia. If it 



