80 CHEMOTAXIS AND CHEMICAL DISINFECTION 



cent, mercuric chloride attracts bacteria by reason of the potash it con- 

 tains, but they rush into the tube only to meet their death from the mer- 

 cury salt. It would therefore seem that chemotaxis, useful as it must be to 

 bacteria in the search for food, may lead them to destruction, although of 

 course they are not exposed in nature to the temptations of such fatally 

 seductive capillaries. 



If the facts gained by these experimental observations are to be 

 employed to elucidate the life-history of bacteria in their natural habitats, 

 in water, in the soil, or in the tissues of the diseased body, a number of con- 

 siderations must be borne in mind. In the first place, chemotaxis can only 

 take place in media which permit free movement, in liquids. Secondly, 

 different kinds of bacteria by no means react in the same way to the same 

 substances. Furthermore, it must be remembered that the sphere of 

 influence of such a capillary is very small. It is not possible to entice 

 into it all the bacteria swimming in the drop. It would not be possible, 

 even if we could renew the solution in the capillary as fast as it diffused so 

 that it could continually give off a diffusion-stream, as perhaps the decaying 

 flocculi in stagnant water do. For the bacteria would be already stimulated 

 by the diffused substance, and to enable them to again react chemotactically 

 a much higher concentration would be necessary. Pfeffer has shown that 

 'Weber's Law' (Psycho-physical law of Fechner), which formulates the 

 relation between strength of stimulus and intensity of sensation, holds 

 good for the chemotactic movements of bacteria. Weber's law is to the 

 effect that an external stimulus just sufficient to give rise to a sensation 

 must be increased in a definite ratio in order to awaken that sensation 

 again. For instance, a weight of one ounce laid upon the hand gives rise 

 to the sensation of pressure upon a certain spot. If now upon the weight 

 a further load of y^ oz. be laid no further sensation is aroused. Not 

 until the original weight has been increased by one-third (i.e. to 1-33 oz.) is 

 the sensation of pressure aroused again. .Had ten ounces been the weight 

 originally employed it would have had to be raised to 13-33 ounces. For 

 thermal stimuli the increase must amount to ? V tn J an d f° r visual stimuli to 

 T J^th of the original stimulus in order that it should again cross the threshold 

 of sensation and enter our consciousness. Precisely the same law governs 

 the phenomena of chemotaxis. In the case of one of the putrefactive 

 bacteria the stimulus has to be increased five times before chemotactic 

 movements are again set up. If the organism be already in a o-i per 

 cent, solution of extract of meat the capillary must contain a 0-5 per cent, 

 solution, if it be in 1 per cent, the tube must be filled with 5 per cent, 

 bouillon before a reaction follows, while to start very active movement we 

 should have to fill the tube with a broth ten or twenty times as strong as the 

 surrounding fluid. These facts should not be forgotten when we attempt 

 to explain by chemotaxis the behaviour of bacteria in the living body, or the 



