ON PHYSICAL OPTICS. 341 



inflammable gas emitted by the coal, which would be made to explode if 

 it came near the flame of a candle. 



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

 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 emis- 

 sion is interrupted by cold. The Bolognan phosphorus was one of the first 

 of these substances that attracted notice ; it is a sulfate of barytes, found 

 in the state of a stone ; it is prepared by exposure to heat, and is after- 

 wards made up into cakes : these, when first placed in a beam of the sun's 

 light, and viewed afterwards in a dark room, have nearly the appearance 

 of a burning coal, or a red hot iron. Burnt oyster shells, t 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 experi- 

 ments apparently accurate, made by different persons, there is reason to 

 conclude that some of these phosphori emit only the same kind of light as 

 they have received, while others exhibit the same appearances, 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 efficacious in exciting 

 in a diamond the appearance of another kind of light, which it was natu- 

 rally most disposed to exhibit. The application of heat to solar phosphori 

 in general expedites the extrication of the light which they have borrowed, 

 and hastens its exhaustion ; it also produces, in many substances, which 

 are not remarkable for their power of imbibing light, a temporary scintil- 

 lation or flashing, at a heat much below ignition : the most remarkable 

 of these are fluor spar in powder, and some other crystallized substances. 

 It appears that luminous bodies in general emit light equally in every 

 direction, not from each point of any of their surfaces, as some have 

 supposed, but from the whole surface taken together, so that the surface, 

 when viewed obliquely, appears neither more nor less bright than when 

 viewed directly.^ 



However light of any kind may have at first originated, there is reason 

 to believe that the velocity with which it passes through a given medium 

 is always the same. It has been ascertained by the astronomical observa- 

 tions of Roemer and of Bradley, that each ray of light, emitted by the 

 sun, arrives at the earth in eight minutes and one eighth, when the earth 

 is at its mean distance of about 95 millions of miles. Roemer deduced 

 this velocity from observations on the eclipses of the satellites of Jupiter, 

 and Bradley 1 1 confirmed it by his discovery of the cause of the apparent 

 aberration of the fixed stars. 



* See Cellio, La Pietra Bolognese Preparata, Rom. 1680. Beccari de Phospho- 

 ris, 4to, Bolog. 1744. T. Wedgwood, Ph. Tr. 1792, p. 28. 



f See Bartholinus de Luce Animalium, 1669. Boyle on the Light of Fish, &c. 

 Ph. Tr. ii. 581, 605, 1672 ; vii. 5107. Works, iii. 304. Canton, Ph. Tr. 1768, 

 p. 337 ; 1769, p. 446. Hulme, ibid. 1800, p. 161 ; 1801, pp. 403, 426. 

 t Hauksbee, on the Production of Light from Phosphorus in vacua, Ph. Tr. 

 xxiv. p. 1865. 



Hist, et Mem. x. 399, Ph. Tr. 1677, xii. 893. 



|| Ph. Tr. 1728, xxxv. 637. 



