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however is by no means an exhauflion but onlya fufpenfion of the luminous property, for asthe dark part of the tube begins to acquire thetemperature of the furrounding air its light willreturn, and it will continue to Ihine longer thanthe other part of the tube in proportion to thetime that it was kept in the freezing mixture.Hence it may be inferred, that at a very lowtemperature the prefent clafs of folar pholphoriwould ceafe to exift.

As cold retards fo heat quickens the emiflionof light from phofplioric fub(lances. Thus ifthe tube mentioned in the ialt paragraph, bedipped half way into boiling water, a great in-creafe of brightnefs will be perceived in theimmerfed portion, which in a minute or lei'swill be fucceeded by entire darknefs, while thepart not heated will fliine much longer, thoughwith lefs fplendor.

If any of thefe phofphori after having beenkept in the dark till it is totally extinguiflied, beplaced (without any further expofure to thelight) in the warm hand, its luminoufnefs willbe for a fhort time renewed; if after thewarmth of the hand has ceafed to produce anyeffect, it be placed in boiling water, a furtheremiffion of light will take place, and finally itmay be entirely exhaufted of its light by beingplaced on a metallic plate, heated nearly to red-nefs. Hence it appears that there are twocaufes of the extraordinary brightnefs exhibitedby folar phofphorus when ftrongly heated im-mediately on being introduced into the darkroom; in the firft place there is a greater quan-tity of light actually given out, and, fecondly,the whole is difcharged in perhaps a twentiethof the time in which even a part would beexhaufted at the common temperature.

With regard to the origin of the light inthefe fubftances there are two theories. Ac-cording to the one it is fimply the folar fightwhich has been previoufly imbibed by them;and according to the other it is the inherentfight of the phofphoric fubftance itfelf. Infavour of the firft hypothefis it may be obfervedthat the phofphorefcence cannot take placewithout previous expofure to the fight, and thatthe period of its greateft brilliance is the mo-^ent of its being placed in the dark, whence itregularly and rapidly diminilhes till it is totallyextingujfhed. But the ftrongeft proof of thetruth of this theory is an experiment related by

ecarta, in which he affirms, that thofe phof-p tori which under the ufual circumftancesemit a white light, give out coloured rays in

the dark, correfponding to thofe which they re-ceive from the fun, with the interpofition of aplate of coloured glafs. Many doubts howeverhave arifen as to the accuracy of the fa£t herementioned. Du Fay ’s experiments made be-fore thofe of Becaria, lead to a directly oppofiterefult, the light emitted by the phofphorus be-ing of the fame tint whatever was the colourof the glafs through which the rays palled tothe phofphorus. In like manner M. Bourrioton repeating the experiments of Becaria, en-tirely failed in producing the effects faid by thelatter to have taken place; nor was M. Magel-lan more fuccefsful. Mr. Wilfon has examinedthis part of the fubjedt confiderably at length.,with the following refults. 1 A piece of pre-pared oyfter fliell which by expofure to thecommon light gave out a white light was atdifferent times placed in the folar rays with theinterpofition of a plate of red glafs, and after afew feconds was haftily transferred into thedark clofet, but though the experiment was re-peated feveral times, not the fmalleft approachtowards a red colour was perceived in the lightof the phofphorus. The fame phofphorus wasnow expofed to the different coloured rays ofthe prifmatic fpeCtrum, but whatever was thecolour of the ray in which it was placed, thefame tint of white light was vifible in every ex-periment.

Another piece of phofphorus difpofed tofhine with a red light was now felected and ex-pofed, firft to the common light and afterwardsin fucceffion to the different refracted rays asfeparated by the prifm, when it was found thatthe phofphoric light was much intenfe whenexcited by white light, and by far the leaftwhen excited by red light; the violet rays ex-cited a degree of red light full ten times asgreat as the red rays did. Another Angularcircumftance was, that if the red prilmatic ravswere thrown upon the phofphorus already madeluminous by the violet rays the immediate ef-fect was a remarkable deterioration in the toneof the colour, fo that it was fcarceiy more bril-liant than when excited by the red rays. Inlike manner the red and yellow prifmatic rayswere found to excite the phofphorefcence in apiece of fliell difpofed to emit blue light, <vithmuch more energy than the blue or violet raysthemfelves.

Thefe Angular fadts appear fcarceiy recon-cilable to the hypothefis of the pholphorefcentlight being merely that which has been previ-oufly imbibed from the fun, yet on the othe r

* Wilton on Phofphoru", p. 97.

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