COLLOIDS 517 



agglutinins from sera, so that it is evident that what we have here 

 is a case of the production of a flocculable combination of bacteria 

 and agglutinin, neither component of which is alone flocculable. 



Citric acid in concentrations ranging between one one-hundredth 

 and one eight-hundredth normal produces flocculation. In either 

 greater or less concentrations no flocculation is produced, which is an 

 illustration of the so-called zone phenomenon. 



Like all other suspensions, bacteria are electrically charged, and 

 consequently wander in the electric field. Under all ordinary condi- 

 tions the charge which they carry is negative, from which in their 

 general conduct it might be expected that they would conduct them- 

 selves similarly to arsenic sulphid, which is also negatively charged. 

 This is found to be the case. Their rate of migration is reduced 

 by acids and evidently somewhart increased by alkalies, although 

 very little alkali is necessary to bring about disintegration of the 

 bacteria. 



It has also been found that all colloids are more or less sensitive 

 to light in respect to their migration rates. Bacteria also show this 

 phenomenon, as they migrate notably slower in the light than in the 

 dark. It is quite possible that this reduction in their electrical 

 charge may to some extent be responsible for the bactericidal action 

 of light. 



3. A very interesting application of the principles of colloidal 

 precipitation by electrolytes has recently been made by Loeb. 17 He 

 finds that the eggs of the Fundulus, a small fish, are killed by being 

 immersed in a pure isotonic salt solution, in spite of the fact that 

 they normally develop in sea water. The factor which allows of their 

 development must therefore be sought in some other constituent of 

 the sea water which is absent in the pure salt solution. This Loeb 

 finds in the presence of small amounts of calcium salts. Further, 

 if a small amount of calcium chlorid be added to the pure salt solu- 

 tion the eggs will develop in it as well as in sea water. Loeb's ex- 

 planation is simple and very ingenious. He supposes that the sodium 

 chlorid is toxic, provided it can diffuse into the egg. In the absence 

 of calcium salts such diffusion is possible because the sodium chlorid 

 is not a sufficiently powerful colloid precipitant to make the mem- 

 brane about the egg impervious. It is, however, very well known 

 that calcium salts (and all bivalent cathions) are far more effective 

 colloid precipitants than sodium ions. Consequently the presence 

 of a relatively small amount of calcium chlorid in the salt solution 

 is sufficient to so condense the egg membrane as to make it impervious 

 to the sodium chlorid, and thus render the latter non-toxic. 



4. Another interesting application of colloidal principles is 

 found in what is known as the Danysz phenomenon. It is found 

 that the neutralization of the toxicity of diphtheria toxin by the 



17 Am. J. Physiol, 6, 411, 1902. 



