Intelligence and Miscellaneous Articles. 11 



rather too small, for two reasons : firstly, because flaws reduce T 



more than - — ^^ (as I explained in the paper) ; and, secondly, 

 ds 



because the measured value of T depends on the cohesion between 



the least-paired particles, whereas E2- depends on that between 



those which are most paired. c ' s 



With liquids the second of these causes of error will be more 

 marked than with solids, because the pairing in them is so much 

 more pronounced ; so that although the measured values of T for 

 liquids are probably much lower than for solids, the true values for 

 electrical stress are likely to be the same for both, or even higher 



dV 



for liquids. It is therefore significant that max ' is so nearly the 



U/S 



same in the three substances, glass, water, and oil, for which I 

 measured it ; for constancy in the values of q and T involves, of 



course, constancy in the values of — mas> (to within the limits 

 of variation of d). cfyS 



I am, Gentlemen, 

 University College, Bristol, Your obedient servant, 



December 15, 1892. A. P. Chattock. 



ON A CHEMICAL ACTINOMETER. BY H. EIGOLLOT. 



This apparatus consists of two plates of oxidized copper im- 

 mersed in water containing y- Vo" °^ chloride, iodide, or bromide of 

 sodium. One of the plates is exposed to the rays of light, the 

 other is protected from the -action either by surrounding it with 

 parchment or paper, or by placing it directly behind the first plate 

 at a distance of about a millimetre. 



The action of the light is instantaneous and disappears when the 

 illumination is destroyed. 



The sensitiveness of an element decreases rapidly at first and 

 then becomes virtually constant. 



The electromotive force of a given actinometer varies with the 

 colour of the light which acts upon it. 



M. BAgolJot has investigated the electromotive force in the 

 different parts of the spectrum, and gives the curves obtained by 

 taking as abscisses the lengths of waves, and as ordinates the 

 divisions read off on the galvanometer-scale. These curves show 

 that the actinometer with copper plates is most sensitive in the 

 red portion of the spectrum, that is in the region most luminous 

 for the eye. In this respect it differs from actinometers with 

 silver compounds, which are more especially sensitive to the most 

 refrangible rays. 



If, after having traversed the spectrum from red to violet, (he 

 system of plates is again exposed in all parts of the spectrum but 

 proceeding from violet to red, the curves retain the same form, 

 and the increase of sensitiveness for red rays observed by 

 M. Becquerel (Lumiere, vol. ii. page 138) for iodized plates which 



