292 INFECTION AND RESISTANCE 



which were positively chemotactic in certain concentrations became 

 neutral or even negative when the concentration was altered. 



We have seen that the action of the leukocytes in moving toward 

 some substances and away from others is entirely analogous to simi- 

 lar phenomena occurring among lower, unicellular forms of life, and 

 the explanations applied to the apparently conscious acts of the 

 ameba, such as the motion toward and the engulfment of food, have 

 been applied to the activities of the leukocytes as well. Many of 

 the theories developed concerning the free living forms, however, 

 have been easily excluded in the case of the leukocytes, because of the 

 environment in which their activities are developed. Thus the many 

 interesting reactions of paramecia and other organisms to light 

 (heliotropism) have little bearing upon this subject, and the views 

 based on the theories of orientation may be excluded on the ground 

 of the symmetry of the normal leukocyte. The observations of 

 Garrey, 41 that indicate that it is the dissociated ions of various acids 

 and bases which are responsible for the directive stimuli exerted 

 upon certain flagellates, may yet result in throwing some light upon 

 leukocytic movements, especially if we can come to accept the con- 

 ceptions of ion-proteins upheld by Loeb 42 and his pupils. However, 

 the facts concerning these phenomena, as well as the possibility, 

 previously mentioned, of the opposite electrical charges carried by 

 the leukocytes and the substances attracting them cannot be regarded 

 at present as more than interesting thoughts. Of more than merely 

 speculative interest, however, are the views of chemotaxis which are 

 based upon the study of conditions of surface tension. In order to 

 consider these properly it will be useful to review briefly the funda- 

 mental principles governing these conditions. 



The molecules of any fluid are held together by mutual attraction 

 due to the force generally spoken of as cohesion. This force is ex- 

 erted by like molecules upon each other in solids more strongly than 

 in liquids, and in gases less strongly. Since we are dealing in this 

 connection with occurrences taking place in liquids, we will restrict 

 our consideration to these. The force of cohesion is influenced in 

 a number of ways. Thus, for instance, heat reduces it, and this 

 is the cause that solids are converted into liquids and liquids into 

 gases, provided of course that the heat brings about no chemical 

 change. In large masses of fluids the force of gravitation over- 

 comes that of cohesion and larger masses of liquid assume the shape 

 of the containing vessel. In smaller masses the force of cohesion 

 tends to bring about the spherical shape. This comes about in 

 the following way: Within the interior of a drop of liquid all the 

 molecules attract each other, and since the force of attraction is 

 equal in all directions it neutralizes itself, and the molecules are 



41 Garrey. Am. Jour, of P~hys., 3, 1900. 



42 Loeb. Am. Jour, of Phys., 3, 1900. 



