Prof. Draper on the Chemical Action of Light. 383 



tlirown about in all directions, crossing and recrossing the liquid 

 in every May by the numerous reflexions they undenvent, — the 

 mercury, as it applied itself to the oviter sm-face of the glass, 

 acting like a spherical concave mirror, and fi'om its mass and 

 high conducting power effectually keeping the temperature down. 

 The quantity of oxygen emitted in a given time was then mea- 

 sured. The same experiment was repeated with the bulb re- 

 moved from the mercury, and the light being sensibly equable 

 after the close of the same period of time ; on measuring the 

 oxygen set free, it was found that the reflecting action of the 

 mercuiy had nearly tripled the effect. 



The power of an incident ray thus depending on the degi-ee 

 of absorption exerted upon it, I was led to inquire whether, by 

 admixtm-e with other suitable substances in a solution vmder- 

 going decomposition, the effect could be increased. Chlorine- 

 water decomposes more rapidly as its yellow tint is deeper. Foiu" 

 equal matrasses were therefore taken : a, containing chlorine- 

 water ; b, the same deepened ^vith chloride of gold ; c, chlorine- 

 water with commercial muriatic acid of a yellow tint ; d, chlorine- 

 water with tinctm-e of iodine. These were exposed altogether to 

 the sun. It was at once obvious that a was giving most oxygen ; 

 and eventually it was found that b yielded a much smaller quan- 

 tity, and c and d none at all. The presence of all these bodies, 

 therefore, exerted a prejudicial effect. 



Any artificial coloration, by the addition of extraneous bodies, 

 does not increase the rate of decomposition, but retards it. This 

 is ])recisely what ought to be expected. A compound atom has 

 its grouping destroyed by the action of light upon its own parts, 

 and IS in no manner concerned in what is taking place in other 

 atoms around. They therefore cannot increase the effect on it. 

 But, on the contrary, they may greatly diminish the action on 

 the mass by exerting a special absorption themselves. Thus the 

 chloride of gold retards the decomposition of chlorine-water when 

 mixed therewith, in the same manner as if it were placed in a 

 trough in front of the water and intercepted the impinging 

 beam. 



Experiments similar to the foregoing were made with a solu- 

 tion of peroxalate of iron mixed with alcohol, ammonio-citrate of 

 iron, tincture of turmeric, chloride of sodium, &c. In every 

 instance it was clear that the action of the light was strictly 

 molecular, that it is impressed on the group of atoms affected 

 and not on the mass ; and that when various bodies are conjointly 

 exposed to the sun, each one undergoes its own specific change, 

 independently of, and unaffected by, all the rest. 



Ten years ago I made many attempts to increase the sensi- 

 tiveness of ordinary photographic paper formed of chloride of 



