Qhfervatiotit on the Chemical ASlmt of Light and Heat, 445 



II. 



On the Oymion or Inference that the Chemical JBians of Light and Heat are the fame. 



By Mr. R. Harrup. 



To Mr. NICHOLSON. 

 SIR, 



JL HE refult of fome experiments which I made in 1797 and 1798, on the efFefts of light 

 upon the mercurial oxides, and which were publiflied lad year in the 5th volume of the 

 London Medical Review and Magazine, was, — That the red oxide of mercury, red ni- 

 trated mercury, and calomel, are reduced by expofure to the rays of the fun. 



That this effeO. takes place when the oxides are inclofed in glafs tubes hermetically fealed, 

 and that the two former are intirely reduced, the tubes containing them becoming incrufted 

 ■with quickfilver, and the latter partially, &c. Struck with the (imilar efte£ts of light and 

 heat in thefe inftances, I put the queftion, if the light which is prefent during the reduc- 

 tion of calcined mercury in the ordinary way, has not fome fhare in producing the efFedt ? 

 I did not know at that time, nor indeed till very lately, that any doubts were entertained 

 of the a£lion of light independent of heat ; and it was only a few weeks ago, whenjooking 

 over the 2d volume of your interefting Journal, that I became acquainted with the opinion 

 of Count Rumford on the fubjefl, " That all the vifible changes produced in bodies by 

 expofure to the a£tion of the fun's rays, are efFe£led, not by any chemical combination of 

 the matter of light with fuch bodies, but merely by the heat which is generated or excited, 

 by the light which is abforbed by them," feems to me to be a conclufion not warranted by 

 experiment. 



Into a very tranfparent glafs tube I put ten grains of red oxide of mercury, fpread out 

 thin on a flip of window glafs. To one half the dire£l rays of the fun were admitted 

 through the tube, and the light excluded from the other half by means of a cafe of pafte- 

 board, and two dices of cork, which fitted the tube exa£tly. That portion to which the 

 light was admitted began to acquire a deeper colour almofl: inllantaneoufly ; fome hours 

 after, it had become of a dark dirty brown colour, with fomewhat of a leaden hue. Upon 

 examination in a flrong light, with a lens the whole furface was found to be covered with 

 fmall metallic globules. The effedl was intirely fuperficial, for at the fmalleft depth bcloAV 

 the furface the oxide was without change. 



After expofure for three days no farther change took place, excepting that the colour 

 was become fomewhat deeper, and the mercury, as it was revived, fublimed and adhered 

 to the upper fide of the tube, which was, at leaft, half an inch difl;ant. The portion of 

 the oxide from which the light was excluded had fuffered no change, excepting a very thin 

 ftratum of it at the end of the glafs next the flicc of cork, which was ufcd as a (lopper. — 



This 



