310 EEPOBT— 1886. 



tliose electrolytes -wluch transmit radiation with the greatest facility, as evidenced 

 hy the effect produced upon a thermopile, are also the worst conductors of electri- 

 city. I may say at once that this was undoubtedly not the fact, and the relation 

 which was supposed to he possible does not exist. 



Considerable time and care were bestowed upon the experiments, and every 

 precaution was taken with the view of ensuring accuracy. The thermopile used 

 was a dehcate one, containing 54 bismuth-antimony pairs ; it was enclosed in a 

 flannel-covered box, fitted with a pane of glass l-.Tmm. thick opposite the face of 

 the pile. Tbe galvanometer was an astatic reflecting instrument of •645 ohm 

 resistance made by Elliott. The source of radiation was a small paraffin lamp, 

 having a glass chimney about 2 mm. thick. 



The parallel glass sides of the cells, used to contain the hquids, were 13 mm. 

 apart, and 1'9 mm. in thickness. A screen, with a small aperture, which could be 

 instantly opened or closed by a sliding shutter, was interposed between the cell in. 

 use and the lamp. The various solutions were in every case exposed to radiation 

 for a period of 30 seconds, and each observation was checked and standardised by 

 the aid of a certain cell containing water. It is hardly necessary to describe the 

 arrangements and method of observation in greater detail. 



A few of the results, which, for the most part, need no comment, are given in 

 the following table : — • 



Solutionis, &c. Diathermancy. 



Empty cell 

 Water distilled 

 ,, from tap 

 Alum, saturated solution 

 Ammonium chloride solution 

 Zinc sulphate sol. sp. gr. 1'157 

 Sulph. acid 1-032 (5 per cent.) 

 „ „ 1-225(30 „ ) 

 „ „ 1-638(72 „ ) 



1000 

 197 

 200 

 204 

 215 

 207 

 208 

 216 

 292 



The observations with sulphuric acid are the most instructive. It is well known 

 that the electrical conductivity of sulphuric acid at firist increases with the concentra- 

 tion, reaching a maximum when the strength of the solution is about 30 per cent., 

 and afterwards rapidly diminishing. That there is no corresponding minimum 

 diathermancy appears clearly enough from the above table, which seems to furnish 

 a conclusive answer in the negative to the question proposed. 



The diathermancy of a solution of zinc sulphate was almost independent of its 

 streng-th, being nearly the same for a 5 per cent, as for a saturated solution. It 

 was quite unaftected by the passage through the solution of a strong battery 

 current. 



Translation of Letters received from Dr. Aerhenius. By Oliver Lodge. 



POLYTECHNICUM, RiGA, Mat) 17, 1886. 



Deae Sie, — I have been much interested in your electrolysis memoir, and since 

 it seems intended to open a discussion, I beg to be allowed to express my views on 

 a few important questions there touched on, especially concerning wandering of 

 ions. 



As I have shown, and a little later also Bouty, one must regard all positive ions, 

 in extremely dilute solutions, as possessing nearly equal velocities ; and in the same 

 way also all negative elements as having an equally great velocity among themselves. 

 It is very probable that these two velocities are also equal to each other for those 

 salts which, by reason of excessive dilution, already approximate closely to the 

 ideal condition ; that is to say, the best conducting salts like KCl, NH^Cl, &c.,. 

 whose ions have nearly equal velocities. 



One could best represent the ions to oneself as spheres of about equal size- 

 (though of unequal weight) which are urged through a resisting medium with the 

 same force, and which very soon attain their terminal velocity. But if the motion 



