VI] OF MACALLUM'S EXPERIMENTS 459 



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 apphcation of it; for the 

 substances which have such action as we are looking for, and 

 which are also actual or possible constituents of the cell, are very 

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

 their precise distribution and locahsation. But in one or two cases 

 we have such means, and the most notable is in connection with 

 the element potassium. As 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 characteristic black precipitate of 

 cobaltic sulphide f. 



By this means Macallum demonstrated, years ago, the unexpected 

 presence of potassium (i.e. of chlorides or other potassium salts) 

 accumulated in particular parts of various cells, both sohtary cells 

 and tissue cells J ; and he arrived at the conclusion that the localised 

 accumulations in question were simply evidences of concentration of 

 the dissolved potassium salts, formed and locaUsed in accordance 

 with the Gibbs-Thomson Law. For potassium (as we now know) 

 has a much higher ionic velocity than sodium ; and accordingly the 



* On the distribution of potassium in animal and vegetable cells, Journ. Physiol. 

 XXXII, p. 95, 1905. (The only substance at all likely "to be confused with potassium 

 in this reaction is creatine.) 



t The reader will recognise a fundamental difference, and contrast, between 

 such experiments as those of Macallum 's and the ordinary staining processes of 

 the histologist. The latter are (as a general rule) merely empirical, while the former 

 endeavour to reveal the true microchemistry of the cell. "On pent dire que la 

 microchimie n'est encore qu'a la periode d'essai, et que I'avenir de I'histologie 

 et specialement de la cytologic est tout entier dans la microchimie": A. Prenant, 

 Methodes et resultats de la microchimie, Journ. de VAnat. et de la Physiol, xlvi, 

 pp. 343-404, 1910. There is an interesting paper by Brunswick, on the Limitations 

 of microchemical methods in biology, in Die Naturwissenschaften, Nov. 2, 1923. 



X It is always conspicuously absent, as are chlorides and phosphates in general, 

 from the nuclear substance. 



