2l'l PHILOSOPHICAL TRANSACTIONS. [aNNO 1746. 



perty they greatly resemble the Bolognian stone, and other phosphor! prepared 

 by art. The phosphori gained by torrefaction, as well as that of Bologna, will 

 not imbibe light, while they are warm ; and this last does not appear so luminous 

 when first prepared, as when it has been so some time. 



The natural phosphori do not differ only in the beforenientioned particulars, 

 but also in the colour of the light itself. The light of the natural generally ap- 

 pears either perfectly bright, or somewhat inclining to yellow ; the artificial pro- 

 duces a red, and sometimes a brown light; but there are some exceptions to both 

 these rules. From these different appearances, the author conjectures, that there 

 are two sorts of fire arising from different principles; viz. that in torrefied 

 substances, from a sulphureous, and that of the natural, from a terrestrial 

 principle. 



In observing a piece of lapis tutiae, which was rough and unequal on its 

 convex side, smooth and somewhat polished on the concave; he found, to his 

 surprise, that the rough side was luminous, and the smooth one not. He was 

 very desirous of investigating the cause of this appearance. He remembered 

 that some polished marbles did not imbibe light, or very little, and that at their 

 edges; but, having lost their polish, they admitted and retained it. He there- 

 fore conjectures that bodies, according to the disposition of their surfaces for the 

 reflection of the light, either suffer or prevent its entrance into them. If this 

 position holds good in the reflection, why should it not with regard to the refrac- 

 tion ? our author produces 2 experiments, which he apprehends not foreign to 

 the present purpose, but is yet making others, for his further satisfaction. He 

 exposed a glass bottle full of well-water to the light, and as soon as possible ob- 

 served it in the dark. As he expected, it imbibed no light. On pouring into it 

 some oil of tartar, it became turbid and whitish, from the well-water being 

 usually impregnated with calcareous matter. On observing it then in the dark, 

 after having been exposed as before, it retained enough of a pale light to distin- 

 guish the shape of the bottle. In a bottle of rain-water he dissolved some talc; 

 which stone, by rubbing, will dissolve in water as salts do, without rendering it 

 opaque; to this solution he added oil of tartar, and this mixture was luminous 

 as the preceding. He therefore concludes, that so long as earthy corpuscles are 

 very small, separate, and agreeing in their surfaces with the water in which they 

 float, they readily transmit the light they receive; for which reason it is impos- 

 sible they should retain light enough to appear luminous in the dark. But, by 

 the affusion of the saline principle, the earthy corpuscles unite with the water 

 and salt; and from the union of these principles the mixture grows thick, by 

 which the ready transmission of light is prevented; so that if this mixture is 

 without colour, or any thing metallic, the light will be stopped long enough to 

 be visible in the dark. But if, instead of oil of tartar, we add sugar of lead. 



