320 Intelligence and Miscellaneous Articles. 



it is given by the formula 



n_ (20fl f )'^ 

 "" (20 P ' + xy' 

 By these experiments a' and p' could be measured. We had 



For one bobbin a =1*78 p =6*87 



ForaBunsen a' =0'753 p'=l'00 



Ratio...., -^=2-37 p ~, =6'87 



a' p' 



With its 96 plates in tension our machine was equal as force to 

 225, and as resistance to 655 Bunsen's elements. In quantity it is 

 only equal to 38 elements of the resistance 18. With other group- 

 ings we had the following results : — 



Value of the machine in Bunsen's 



elements. 

 . » s 



Electromotive ResUtance . 



force. 



Tension 226 655 



Two groups of three .... 113 163*6 



Three groups of two .... 75 72*3 



Quantity 37'9 18'0 



In all cases the effect of the machine and its arrangement might be 

 so modified as to produce in a given position of the circuit the maxi- 

 mum calorific effect. — Comptes Rendus, July 1, 1868. 



ON THE DIATHERMANCY OF CHLORIDE OF POTASSIUM. 

 BY G. MAGNUS*. 



There are now found at Stassfurt large crystals of perfect trans- 

 parency to which mineralogists have given the name sylvine. I 

 have ascertained that sylvine has the same properties as rock-salt 

 as regards its transparency for heat. A plate of sylvine 26 millims. 

 in thickness allows 76 per cent, of the incident heat to traverse it, 

 which is the same proportion as a plate of rock-salt of the same 

 thickness from Stassfurt. 



Melloni found that a plate of rock-salt of 2' 6 millims. (that is, a 

 tenth of the thickness of my plate) allowed 92*3 per cent, of the in- 

 cident heat to traverse it ; another plate, which he calls imperfectly 

 transparent (louche) y only allowed 62 per cent, to pass. Thus 

 sylvine and Stassfurt rock-salt, with ten times the thickness, possess 

 a diathermancy which is the mean of those which Melloni found for 

 plates of rock-salt. 



I had occasion to try a plate of rock-salt from Wiirtemberg, which 

 gave 92-6 for a thickness of 26 millims. 



The diathermancy of sylvine resembles that of rock-salt in another 

 respect : it does not vary with the temperature of the source. I 

 have found it the same for heat emitted by a lamp and for that arising 

 from a vessel heated to 100°. Rock-salt has hitherto been the only 

 substance known to be endowed with this property. — Comptes 

 Rendus, June 29, 1868. 



* Extract from a letter to M. Regnault. 





