THE AGGLUTININS 213 



which is generated wherever a fluid comes into contact with any 

 other substance, whether solid, liquid, or gas, and which acts 

 exactly as if the surface of the fluid in question were in a state of 

 tension, like a stretched film of indiarubber. If a relatively 

 small amount of any fluid be suspended in another fluid of the 

 same specific gravity with which it does not mix, it will assume 

 the form of a sphere : this is because the sphere has a smaller 

 surface for a given volume than any other solid body, and the 

 hypothetical film on the surface continually contracts until this 

 figure is assumed. Hence leucocyte's, and most other free cells 

 consisting of fluid or semi-fluid protoplasm, tend to assume a 

 spherical form when in a resting condition ; hence also, of course, 

 the spherical form of soap-bubbles, oil-globules, etc. Now consider 

 the case of two spheres acted on by surface tension and just 

 touching one another ; for example, take two drops of oil 

 suspended in a fluid of about the same specific gravity. If we 

 regard the surface of the two spheres as continuous, it is obvious 

 that it is much larger than it would be if the two drops coalesced 

 to form a single sphere. (It is roughly larger in the proportion of 

 4 : 3.) The film, therefore, will contract until the two globules are 

 drawn into a single drop, with double the volume of each original 

 globule, but with a much smaller superficies than that of the two 

 separately. This process will take place whenever two bodies, 

 neither or both of which are wetted by the fluid, are brought in 

 contact or very close together : when one is wetted and the other 

 not, they tend to repel one another. The force of surface tension 

 only extends for an exceedingly minute distance into the fluid 

 from the surface, and therefore does not draw the substances 

 together if they are a finite distance apart. Its action comes into 

 play when the two bodies touch one another in one point, so that 

 the surfaces between the two bodies and the fluid join to become 

 one at this point. Thus, if two red blood-corpuscles touch one 

 another obliquely at one point, they become drawn together, and 

 slide the one on the other until they oppose as small a surface as 

 possible to the surrounding fluid. This, of course, is when the one 

 lies flat on the other, as in rouleaux formation. Two wooden 

 discs enclosed in a small indiarubber bag would act precisely 

 similarly. 



The exact way in which the agglutinin affects the surface 

 tension between the bacteria and the fluid in which they lie is not 

 quite clear, and raises difficult questions in molecular physics, some 



