64 OUTLINES OF BACTERIOLOGY 



repulsion the bacteria will recede from the capillary tube as far as 

 possible. By this method the attraction or repulsion for bacteria of 

 various substances has been investigated. In general it may be stated 

 that all metallic salts exert attraction, the most powerful being those 

 of potassium, sodium, rubidium, calcium, barium, and strontium. 

 Thus a 2 per cent, solution of common salt (sodium chloride) exerts 

 a powerful attraction. On the other hand, all salts with an acid 

 reaction repel bacteria, and the same may be said of all free acids, 

 free alkalies, and alcohol. Among organic substances, peptone and 

 asparagin in weak solutions are strongly positive, carbohydrates, e.g. 

 sugar, are weakly positive, whilst glycerine is neutral. A very essential 

 point to notice is that a substance which is positive in a weak solution 

 may act as a repellent in a stronger solution, this being the case with 

 peptone and asparagin for example. Whilst a 2 per cent, solution of 

 common salt is positive, 19 per cent, and stronger solutions are entirely 

 negative. The same holds for most of the other substances that are 

 positive in their action. 



A peculiar fact in connection with their movements is, that bacteria 

 are subject to JFeber's Law. To explain this law, suppose that a weight 

 of 1 lb. be placed on the hand. It has been found that it requires the 

 addition of J lb. before the hand feels that an additional weight has 

 been placed on it. If 2 lbs. be placed on the hand instead of 1 lb., it 

 will now be found that J of 2 lbs., that is f lb., will have to be added 

 before the hand feels that an additional weight has been placed on it. 

 That is to say, the weight that must be added before an addition is 

 felt, is always ^ of what is already on the hand. This is expressed by 

 saying that the perception-coefficient is ^. That bacteria obey this law 

 may be seen from the following experiment. A small piece of a 

 capillary tube is filled with a 1 per cent, salt solution in which some 

 motile bacteria are contained, and another tube with a ^ per cent, salt 

 solution containing the same bacteria. If, now, both tubes be placed in 

 a drop of a 3 per cent, salt solution, the latter exerts a chemiotactic 

 attraction for the bacteria in the tubes. But whilst the bacteria in 

 the tube containing the i per cent, salt solution will move out of the 

 tube, those in the other tube will not do so. The reason of this is that 

 th.B perception-coefficient for bacteria is 5, hence, to ensure movement, the 

 percentage of salt in the drop must be at least 5 times that of the salt 

 in the tube. The salt in the drop is 6 times more concentrated in 

 the case of the one in which movement of the bacteria takes place, but 

 only 3 times more concentrated in the case of the other, so that move- 

 ment does not take place. 



