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Show r 16 J IV. "EFFULGENT MAins! you round deciduous day, Treffed with [oft beams, your glittering bands array ; On Earth's cold bofom, as the Sun retires, Confine with folds of air the lingering fires 0' er Eve's pale forms diffufe phofphoric light, And deck with lambent flames the {hrine of Night. Co>~fint with folds of air. 1. 176. The air, like all other bad conduttors of elecbicity, is known to be a bad conduttor of heat; and thence prevents the heat acquired from the fun's rays by the earth's furfacc from being fo foon diffipated, in the fame manner as a blanket, which may be confidered as a fponge filled with air, prevents the efcape of heat from the perfon wrapped in it. This feems to be one caufe of the great degree of cold on the tops of mountains, where the rarity of the air is greater, and it therefore becomes a better conduetor both of heat and elechicity. ~ See note on Barometz, Vol. II. of this work. There is however another caufe to which the great coldnefs of mountains and of the bigher regions of tbe atmofphere is more immediately to be afcribed, explained by Dr. Darwin in the Philo{. Tranf. Vol. LXXVIII. who has there proved by experiments with the air-gun and air-pump, that when ·any portion of the atmofphere becomes mechaninically expanded, it abforbs heat from the bodies in its vicinity. And as the air which creep~ along the plains, expands itfelf by a part of the preffure being taken off when it afcends the fides of mountains; it at ~he fame time attraCts heat from the fummits of thofe mountains, or other bodies which happen to be immerfed in it, and thus produces cold. Hence he concludes that the hot air at the bottom of the Andes becomes temperate by its own rarefaction when it afcends to the city of ~ito ; and by its further rarefaction becomes cooled to the freezing point when it afcends to the fnowy regions .on the fummits of thofe mountains. To this alfo he attributes the great degree of cold experienced by the aeronauts in their balloons; and which produces hail in fummcr at the height of only two or three miles in the atmofphere. Diffufe phojphoric light. 1. 177. I hav.eoiten been induced to believe from obfervation, 'that the twilight of the evenings is lighter than that-of the mornings at the fame diflance [ 17 )' So, warm 'd and kindled by meridian fk.ies, And view'd in darknefs with dilated eyes, BoLOGNA's chalks with faint ignition blaze, BEccARI 's fhells emit prifmatic rays. 180 :rom no~n. Some may afcribe this to the g~eater height of the atmofphere in the evenI~ gs hav1~g ?e_en rarefied by the ftm during the day; but a5 its denfity mufl: at the fame tnnc be dnmmfhed, its power of refraCtion would continue the fame. I fhould rather fuppof~ that it t~ay be owing to the phofphorefcent quality (as it is called) of almoft all bod1es; th:lt Is,_ when they have been expofed to the fun, they continue to emit light for a confiderable time afterwards. This is generally believed to arifc either from fuch bodies giving out the light which they had previoufly abforbed; or to the continuance of a flow combufl:ion which the light they had been previoufly expofed to had excited. See the next note. _ Brccar~'s f!Jells. I. 182. Beccari made many curious_ experiments on the phofphoric light, as It rs called, which becomes vi!lble on b0dies brought into a dark room, after having been previoufly expofed to the funfhine. It appears from thefe experiments that almofl: all inflammable bodies poffefs this quality in a greater or lefs degree ; white paper or linen thus exam ined after having been expofed to the funfhine, is luminous to an extraordinary degree; and if a perfon fhut up in a dark room, puts one of his hands out into the fun's light for a fhort time and then retracts it, he will be able to fee that hand difl:inetly, and not the other. Thefe experiments fecm to countenance the idea of light being abforbed and again emitted from bodies when they are removed into darknefs. But Beccari furthe r pretended, that fome calc;trcous compofltions when expofed to red, yellow, or blue light, through coloured glaffcs, would on the-ir being brought into a dark room emit coloured lights. This mifl:akcn faa of Beccari's, Mr. Wilfon decidedly refutes ; and among ma;1y other curious experiments difcovered, that if oyfl:crfhells were thrown into a common fire and calcined for ,Jbout half an hour, and then brou ght to a perfon who had previoufly been fomc minutes in a dark room, that many of them would exhibit beautiful irifes of prifmatic coloms, whence probably arofe Reccari 's mifl:ake. Mr. Wilfon hence contends, that thefe kinds of phofphori do not emit the light they had previoufly received, but that they are fet on fire by the fun 's rays, and cor11inue for fome time a flow combuflion after they are withdrawn from the light. Wilfon's Experiments on Phofphori. Doclfley, 1775· The Bolognian Hone is a felenite, or gypfum, and has been 'ong celebrated for its phofphorefcent quality after having been burnt in a fulphurous fire ; and cxpofed when D |
