

Chemistry and Physics. 309 



SCIENTIFIC INTELLIGENCE. 



I. Chemistry and Physics. 



1. On Luminous and Chemical Energy. — The radical distinc- 

 tion which exists between photochemical reactions which are 

 endothermic, in which light energy is converted into chemical 

 energy, and those which are exothermic, in which the light is 

 merely auxiliary, the greater portion of the change being effected 

 by purely chemical agencies, has been recently emphasized by 

 Berthelot, who has pointed out that it is only the former class 

 of reactions which are adapted for use in the measurement of 

 luminous energy. He has studied a number of substances when 

 exposed to direct sunlight, to diffused daylight, or when kept in 

 the dark, with reference to the changes thereby produced. The 

 substances were contained in sealed tubes and the effect of dif- 

 ferent wave-lengths upon them was determined by surrounding 

 them with colored media. Nitric acid when concentrated is 

 decomposed by light at ordinary temperatures, and possibly at 

 still lower temperatures, the reaction being similar to that which 

 takes place in the dark at 100°, and which yields nitrogen per- 

 oxide, oxygen and water. The blue rays are most effective in 

 producing the action, which is endothermic and not entirely 

 reversible. The diluted acid is not affected by light, that of 

 density 1*365 undergoing no change after weeks of exposure. 

 The nitrogen peroxide formed tends to absorb those rays which 

 are active in producing it. Iodic acid and oxide, while stable in 

 diffused daylight, are decomposed by direct sunlight, setting free 

 iodine and oxygen ; the reaction being analogous to that produced 

 by heat at 300°. It is limited by the absorptive action of the 

 iodine vapor and by the deposition of iodine on the surface of the 

 compound. Hence, while endothermic, this reaction is not suit- 

 able for the measurement of light energy. The action in the case 

 of hydrogen iodide is on the line limiting endothermic and exo- 

 thermic reactions, though the author's view is that a small amount 

 of heat (0-4 calory) is evolved ; though when solid iodine is set 

 free, it is decidedly exothermic, 6 4 calories being evolved. The 

 blue and violet rays are most effective, and the decomposition is 

 most rapid in direct sunlight, there being an intermediate pro- 

 duction of hydrogen periodide (HI 3 ?). Hydrogen bromide is not 

 affected by the action of light even at 100°. Nor was any action 

 observed with C0 2 either alone or mixed with hydrogen, with CO 

 alone or mixed with hydrogen or oxygen, with S0 2 or HN0 3 

 when mixed with hydrogen. Red mercuric oxide is more rapidly 

 decomposed than the yellow variety. Mercurous oxide when 

 exposed to light yields traces of metallic mercury. No action 

 was noticed with lead peroxide, silver oxide, or with mercuric 

 chloride, bromide or iodide. Hence the author considers that the 

 measurement of luminous energy by transformation into chemical 



