156 Dr. C. Chree on Denudation and Deposition. 



0"0l volt in excess of their proper value they were found to 

 recover almost immediately. 



The conclusions drawn by the author from his experiments 

 are that, so far as regards the fulfilment of the conditions 

 necessary for a standard of E.M.F., the cadmium cell is 

 distinctly superior to any modification of Clark cell. The 

 effects of all ordinary impurities are very small, especially the 

 effect of acid salts, which, although they slightly increase the 

 temperature coefficient of the cell, apparently do not affect it 

 in any other way. The effect of added acid, however, in- 

 creases the E.M.F., and generally makes the cells untrust- 

 worthy. 



Cadmium cells should not he subjected to large and sudden 

 variations of temperature on account of the large value of 

 the time-lag. As regards the type of the cell employed, the 

 author considers the crystal form the most satisfactory, both 

 on account of its portability and because the liquid in the cell 

 is sure to be saturated at all temperatures owing to the 

 excess of CdiS0 4 crystals. 



XIII. Denudation and Deposition. 

 To the Editors of the Philosophical Magazine. 

 Gentlemen, 



IN your June number, Dr. G. J. Stoney, replying to some 

 criticisms of mine, seems anxious to largely widen the area 

 of our differences. I fear, however, that I cannot allow myself 

 the luxury of a discussion on such an interminable subject as 

 the limits of our ignorance of the earth's physical constitution. 

 My original criticisms were written in the belief that certain 

 numerical results put forward by Dr. Stoney were obtained 

 by applying the ordinary mathematical theory of elasticity 

 to the earth. The results seemed to be based on an elastic 

 constant, the compressibility of glass, the value of which is 

 quoted on p. 373 ; and the remarks on p. 375 seemed to say 

 that while slow plastic changes were believed to exist, they 

 were, lor the time being, left out of account. 



My criticisms referred to the serious obstacle that exists to 

 the application of the ordinary mathematical theory of elastic 

 solids to the earth, when one supposes the material so com- 

 pressible as Dr. Stoney's figures made it. The expressions 

 found for the gravitational strains are so large as to violate 

 the fundamental hypothesis on which the mathematical theory 

 is based, viz., that strains are small quantities whose squares 

 are negligible. The figures quoted by me for the change of 

 radius of an elastic solid earth of the compressibility of glass 

 were intended solely to illustrate this point. 



