116 . Mr. Talbot on the Nature of Light. 



posed to the sun's rays, continue to give light when removed 

 into a dark apartment. Some important observations con- 

 cerning them have been lately published by Osann*. He 

 found that no augmentation of their light took place in oxy- 

 gen gas, nor any diminution of it in hydrogen gas ; so that 

 no combustion existed, nor was there any sign of it. But is 

 there any difficulty in supposing that the molecules of a sub- 

 stance, after having been caused to vibrate powerfully by the 

 sun's rays, may afterwards continue to move spontaneously 

 for a certain time when left to themselves? If any doubt 

 should exist as to the possibility of the thing, I think it will 

 be removed by the experiment which I am about to mention. 

 A sheet of paper was moistened with a solution of nitrate of 

 silver, a substance which, it is well known, is capable of being 

 blackened by the influence of solar light. Half of the paper 

 was covered, and half exposed to sunshine ; but owing to its 

 being a dull day in the winter season, no effect was produced. 

 After several minutes the paper was removed, and being ex- 

 amined, showed hardly any perceptible difference between the 

 part that had been covered and that which had been exposed 

 to the sun. It was then removed to another room, where the 

 sun does not shine in the winter season, and accidentally left 

 on a table exposed to the daylight. Some hours afterwards 

 I was surprised to find that the paper had become partially 

 darkened, and that the dark part was that which had been 

 previously but ineffectually exposed to the sunshine, while 

 the other part still retained much of its original whiteness. 

 This anomalous fact, of which I could find no explanation 

 at the time, appears to me now to be closely connected with 

 what I have advanced as a probable cause of phosphorescence. 

 In the first place, white paper is known to be a weak solar 

 phosphorus, and even were it not so, we have here an indu- 

 bitable proof, that in consequence of its exposure to the solar 

 rays a spontaneous action of some sort or other commenced, 

 and continued for some hours. Putting the facts together, I 

 see no great improbability in supposing that the spontaneous 

 action which is sufficient in one case partially to decompose 

 nitrate of silver, may be sufficient in another case to act upon 

 light so as to produce what we call phosphorescence. If this 

 idea is admitted, the cause of the ultimate cessation of phos- 

 phorescence may be, that the vibrating particles at length ar- 

 range themselves in positions of stability; from which, how- 

 ever, they may be again deranged by a new impulse, as, for 

 instance, by an electric shock. Mr. Pearsall has, in fact, dis- 

 covered that extinct phosphorescence is powerfully revived by 

 electric discharges. Moreover, it has been found by experi- 



• Poggendorfs Annals, New Serie3, vol. iii. p. 405. 



