Chemistry and Physios. 73 



du n du n da 



=— 4-2X2-84 for PQ and PR, 



the first differential coefficient being derived from the formula 

 from which fx is calculated, and the second is given on p. 66. 

 From these and the values of fx on p. 70 treated as independ- 

 ent determinations, we have 



23-9 dt= — 2 



11-9 dt= 2 



11-9 dt=-4, 

 whence 



<ft=-0°-084±0-032. 



From this it is obvious that such a correction is required. 

 Supposing, then, that the angles of the prism given above 

 correspond to a temperature of 19 0, 916 C. instead of 20° C. we 

 have the following definitive values for the quantities involved : 



a A A t \x 



PQ 60° 1' 24"-83 52° 4' 10"-20 47° 3' 58"-26 1'658392 



PR 59 57 37-66 51 58 11*52 46 58 45"69 1-658387 



QR 60 57-60 52 3 26-10 36 1 43-06 1-658389 



whence /* =l-658389 ju e 1-486452 



yu' e // e [calc.J 



PQ 1-606113 1-606110 



PR 1-606102 1-606100. 



The conclusion is, that Huyghens's law is probably true to- 

 less than one part in five hundred thousand, and, consequently, 

 that there is no known method by which we can hope to 

 discover an error in it by observation alone. 



New Haven, Nov., 1887. 



SCIENTIFIC INTELLIGENCE. 



I. Chemistry and Physics. 



1. On the Decomposition of the Hydrides of the Halogens by 

 Light in presence of Oxygen. — Some time ago it was observed 

 by Backelandt and by McLeod that hydrogen chloride gas, when 

 exposed to the combined action of atmospheric oxygen and sun- 

 light, was partially decomposed, chlorine being evolved. In 

 order to determine the conditions under which this change is 

 effected, Richardson has made a series of experiments on hydro- 

 gen chloride, bromide and iodide gases, (1) when the gaseous 

 mixture exposed to the sunlight is moist; and (2) when it is dry. 



