288 A NOTE ON ADSORPTION [ch. 



reconsider it, and be prepared to deal with important localised 

 variations in the surface tension of the cell. For, as a matter of 

 fact, the simple case of a perfectly symmetrical drop, with uniform 

 surface, at which adsorption takes place with similar uniformity, 

 is probably rare in physics, and rarer still (if it exist at all) in the 

 fluid or fluid-containing system which we call in biology a cell. 

 We have mostly to do with cells whose general heterogeneity of 

 substance leads to qualitative differences of surface, and hence to 

 varying distributions of surface tension. We must accordingly 

 investigate the case of a cell which displays some definite and 

 regular heterogeneity of its liquid surface, just as Amoeba displays 

 a heterogeneity w^hich is complex, irregular and continually 

 fluctuating in amount and distribution. Such heterogeneity as 

 we are speaking of must be essentially chemical, and the prelimin- 

 ary problem is to devise methods of "microchemical" analysis, 

 which shall reveal localised accumulations of particular substances 

 within the narrow limits of a cell, in the hope that, their normal 

 effect on surface tension being ascertained, we may then correlate 

 with their presence and distribution the actual indications of 

 varying surface tension which the form or movement of the cell 

 displays. In theory the method is all that we could wish, but in 

 practice we must be content with a very limited application of it ; 

 for the substances which may have such action as we are looking 

 for. and which are also actual ^r possible constituents of the cell, 

 are very numerous, while the means are very seldom at hand to 

 demonstrate their precise distribution and localisation. But in 

 one or two cases we have such means, and the most notable is in 

 connection with the element potassium. As Professor Macallum 

 has shewn, this element can be revealed, in very minute quantities, 

 by means of a certain salt, a nitrite of cobalt and sodium*. This 

 salt penetrates readily into the tissues and into the interior of the 

 cell; it combines with potassium to form a sparingly soluble 

 nitrite of cobalt, sodium and potassium ; and this, on subsequent 

 treatment with ammonium sulphide, is converted into a character- 

 istic black precipitate of cobaltic sulphide f. 



* On the Distribution of Potassium in animal and vegetable Cells; Journ. of 

 Physiol. XXXII, p. 95, 1905. 



f The reader will recognise that there is a fundamental difference, and contrast. 



