70 BACTERIA AND SOIL FERTILITY 



solution. When the bacteria get into one of these highly con- 

 centrated drops of poison they are quickly killed. 



(5) The disinfectants of the heavy metal group must be in 

 solution so as to form ions of the metal which can combine with 

 the protein of the bacterial cell and thus cause death. 



(6) Bacteria may withstand the action of a strong poison for 

 a short time, whereas a weaker solution active for a longer time 

 will cause death. 



Some of the more extensively used disinfectants are quicklime, 

 bleaching powder, sulfur dioxid, formaldehyde, and mercuric 

 chloride. That extremely violent poison, hydrocyanic acid, of 

 which a Freshman once said, "It is so poisonous that a drop placed 

 upon the tongue of a dog will kill a man,'' is a very poor disin- 

 fectant, but it quickly kills all insects. 



Chemotaxis. — ^It has been repeatedly shown that bacteria, like 

 other free-moving organisms, are attracted by certain chemical 

 substances in solution (positive chemotaxis), and repelled by 

 others (negative chemotaxis). 



A very simple and eiEcient method of studying this in bacteria 

 is as follows:' A capillary tube, sealed at one !nd and from 5 

 to 10 mm. in length, is filled with a 5 per cent slightly alkaline 

 solution of beef tea or peptone. The outer surface of the glass 

 is carefully cleaned from any trace of the beef tea and is placed 

 in a drop of water containing bacteria. In a few seconds the 

 bacteria are found to thickly congregate around the open end of 

 the tube. This is not due to any volition on the part of the 

 bacteria, nor is it due to their congregating in places most favor- 

 able for life, for had the capillary tube been filled with sugar 

 or glycerine, which are excellent food-stujffs, there would have 

 been no such gathering of the bacteria at the end of the tube. 

 Moreover, a dilute solution of potassium chloride and mercuric 

 chloride attracts bacteria, but they rush into the tube only to 

 meet their death. Some say that bacteria are attracted to the 

 roots of the alfalfa in the soil by some such means, and it is 

 quite certain that the white corpuscles are attracted to an in- 

 f ected wound by this means. 



