BY R. GREIG-SMITH. 143 



and, in the beginning of this research, into the etiology of 

 toxicity, some of the more Hkely bacteria were tested with more 

 or less completeness. The work was tentative, the bacteria were 

 tried one way and another. A favourable result was occasionally 

 obtained, which led to repeated trials with slight variations, but 

 these ended in failure to obtain a truly toxic condition. Some 

 moulds were also tested, but the experiments with these did not 

 lead me to believe that the source of the toxins would be found 

 in the flora of the soil. The fauna remained, and when the 

 amcebae were tested, the first results were so satisfactory that the 

 source of the toxins seemed to have been traced. Unfortunately, 

 these results were not confirmed, and, as the work proceeded, it 

 became evident that the toxic effect was caused by the produc- 

 tion of alkali in the solutions in which the protozoa were growing. 

 The test-bacteria were very sensitive to changes of reaction, and, 

 so far as the solutions were concerned, it was made clear that a 

 perfectly neutral solution was exceedingly difficult to maintain, 

 and that any departure from a strict neutrality retarded the 

 growth of the bacteria and exhibited some of the effects of a 

 toxic solution. The main steps of the work that led to this 

 conclusion are recorded in the following pages. Many experi- 

 ments have been omitted because the results did not appear to 

 justify a lengthening of the paper. They were either indefinite 

 or confirmed a negative result previously obtained. 



It is well known that some soil-bacteria are inimical to others, 

 and it was considered that some one group might be specially so 

 to bacteria generally. Instances of an inhibiting or toxic effect 

 exercised by some micro-organisms against others are familiar to 

 most workers in soil-bacteriology. It is an ordinary experience 

 to find certain colonies of bacteria, such as Bac. mycoides and 

 certain moulds, passing over or through other bacterial colonies 

 when spreading over the surface of an agar-plate. Occasionally, 

 they are seen to avoid some particular colony, and we find these 

 surrounded by a clear zone of agar, across which the wandering- 

 mould or bacterium will not pass. Apparently, the colony has 

 sent into the medium some toxic substance, and there does not 



