ON PHYSICAL OPTICS. ' 4,35 



and animal substances, which appear to be undergoing a slow decomposition,, 

 not wholly unlike combustion. Thus decayed wood, and animal substance* 

 slightly salted, often afford spontaneously a faint light, without any elevation 

 ef temperature ; and it is not improbable that the light of the ignis fatuus 

 may proceed from a vapour of a similar nature. 



The effects, which are commonly attributed to the motions of the electrical 

 fluid, are often attended by the production of light; and violent or rapid 

 friction frequently seems to be the immediate cause of its appearance. But 

 it is diificult to ascertain whether friction may not be partly concerned in 

 the luminous phenomena attributedto electricity, or electricity in the apparent 

 eflf'ects of friction. Light is sometimes produced by friction with a much lower 

 degree of heat than is required for combustion, and even when it is accom- 

 panied by combustion, the heat produced by the union of these causes may be 

 very moderate : thus it is usual in^some coalmines, to obtain a train of light by 

 the continual collision of flint and steel, eflfected by the machine called a 

 fire wheel, in order to avoid setting fire to the inflammable gas emitted by the 

 coal, which would be made to explode if it came near the flanie of a candle. 



There is a remarkable property, which some substances possess in arv 

 eminent degree, and of which few, except metals and water, are entirely 

 destitute. These substances are denominated solar phosphori; besides the 

 light which they reflect and refract, they appear to retain a certain portion, 

 and to emit it again by degrees till it is exhausted, or till its emission is in- 

 terrupted by cold. The Bolagnan phosphorus was one of the first of these 

 substances that attracted notice ; it is a sulfate of bary tes, found in the st.ate 

 of a stone; it is prepared by exposuri to heat, and is afterwards made up into 

 cakes: these, when first placed in abeam of the sun's light, and viewed after- 

 wards in a dark room, have nearly the appearance of a burning coal,or a red hot 

 iron. Burnt oyster shells,and muriate of lime have also the same property, and 

 some specimens of the diamond possess it in a considerable degree. From the 

 different results of experiments apparently accurate, made by difterent 

 persons, there is reason to conclude that some of these phosphori emit only 

 the same kind of light as they have received, while others exliibit the same ap- 

 pearances, to whatever kind of light they may have been exposed. Sometimes 

 .it has even been found that light of a particular colour has been most effita- 



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