4 BACTERIA. 



pure cultures of the tetanus bacillus. Salomonsen (1876) 

 observing that the dark colour which makes its appearance in 

 defibrinated ox blood always commences as minute points, 

 concluded that each of these patches was due to the changes 

 set up by micro-organisms growing at these points. He 

 therefore drew such defibrinated blood into a capillary tube, 

 and by carefully noting the appearances presented in the tube 

 was able to follow the development of a variety of organisms 

 which, when few in number, were separated by considerable 

 intervals of bright red blood in which no organisms could be 

 seen, though in time these gradually ran together. Lister 

 (1878) and Naegeli (1879) obtained their pure cultivations by 

 a kind of fractionating method, in which they distributed a 

 number of organisms into large quantities of fluid by making 

 greater and greater dilutions of their organisms in broth, 

 until the dilution was such that a single drop did not con- 

 tain more than a single organism, and a series of such 

 drops introduced into a number of flasks containing sterilized 

 broth gave a large proportion of pure cultivations. With 

 the pure cultivations so obtained the first really accurate ex- 

 periments in connection with the physiological chemistry of 

 bacteria were carried on. This method, however, though a 

 very great advance on any that had hitherto been devised, 

 was somewhat complicated, and it was open to certain other 

 objections (though it is still most useful in many investiga- 

 tions). It was not until Koch, perfecting Klebs' and Brefeld's 

 gelatine method, was able to " fix " the organisms in situ as 

 it were, in the nutrient medium, that the bacteria could be 

 kept isolated, the resulting colonies studied and then removed 

 to other media for further examination. The method was 

 so simple and so reliable that it was eagerly seized upon 

 by those who were interested in the solution, not of 

 botanical problems merely, but of many questions con- 

 nected with putrefaction, fermentation, and disease that 

 had cropped up so frequently, but which had to be left 

 without a satisfactory answer, and which, in consequence 

 of Davaine's and Pasteur's researches, were again assuming 

 such prominence. 



In the first place, experimenters were able to put to the 

 test the many observations that had been made on both the 

 structure and the function of micro-organisms. Once a pure 

 culture became available, it was an easy matter to determine 



