Intelligence and Miscellaneous Articles. 507 



moment no real difference exists between the two cases ; but that I 

 did not make special mention of this of course displays a want of 

 acuteness on my part, which I regret. The chief point, a " sub- 

 sequent decrease " of the current, is also observed by Mr. Brown. 



Mr. Brown asserts, it is true, that the last facts can be explained 

 as well by Helmholtz's theory as by what he calls his hypothesis, 

 but unfortunately he has not given his explanation. 



This hypothesis is said by Mr. Brown to be opposed to the 

 theory of Helmholtz, of which he says : " It was perhaps unneces- 

 sary to disprove what had not been proved." If a theory can be 

 proved by developing its consequences and comparing them with 

 experiment, the theory of Helmholtz is proved by many facts. I 

 may recall, for instance, the agreement between the electromotive 

 force of a perfect dropping electrode and the value of the E.M.F., 

 which the maximum surface-tension of mercury in contact with 

 the same electrolyte reaches ; further, the fact that this maximum 

 surface-tension of mercury always shows the same value, while 

 the natural surface-tension depends to a great extent upon the 

 nature of the electrolyte in contact with it ; further, the connexion 

 between the currents caused by stretching the common surface and 

 its state of polarization, &c. On the other hand, I am not aware if 

 there exists a single fact in favour of Mr. Brown's hypothesis. 



But Mr. Brown prefers his own theory because it seems to him 

 " much simpler " than Helmholtz's theory. I must, however, con- 

 fess that the assumption that mercury is negatively charged in 

 contact with sulphuric acid is, to my mind, in no perceptible 

 degree simpler than the assumption that it is positively charged. 



Even if Helmholtz's theory were no better proved than Mr. 

 Brown's, I should probably prefer the former. This may be 

 attributed to national prejudice, but I cannot help it. 



Leipzig, November 16, 1890. 



VELOCITY OF SOUND AT VERY LOW TEMPERATURES. 

 BY M. GREELY. 



A base-line of 1279 metres was accurately measured, and the 

 interval determined between the flash of a gun at one end and the 

 appearance of the sound-wave at the other. 



The following results were obtained, where t is the temperature, 

 x the number of observations, and v the corresponding velocity : — 



t =-10-9 -25-7 -37-8 -45-6 



x= 53 114 164 205 



v = 326-1 317-1 309-7 305-6 m. 



The velocity diminishes therefore 0-603 metre for 1° C. — Meteo- 

 rolog. Zeitschrift, vii. p. 6 (1890). 



