210 PHILOSOPHICAL TRANSACTIONS. [aNNO I746. 



surface. But this is most remarkable in the rotten wood of the fir-tree, and 

 some others, where in the dark you see shining spots as large as tares ; whereas 

 in full light the whole surface appears alike. Some few barks are luminous, but 

 not considerably so; but no fniits, seeds, or their meals. Cotton appears very 

 bright, and the crystals of tartar; but fine loaf sugar appears the most luminous 

 of all, both without and within. Gums and resins retain no light. 



There is avast variety of phosphori in the animal kingdom, such as the bones 

 and teeth ; to these may be added the shells of fish, egg-shells, the human cal- 

 culus, bezoar, and in whatever parts of animals the terrestrial pi'inciple is very 

 predominant. But where there is a considerable quantity of oily matter, as ia 

 the hoofs, horns, and feathers, no light is manifest. 



The author, having gone through the natural phosphori, proposes some 

 queries concerning them; of which the first is, in what and how great a light 

 the object ought to be placed ? He tried different phosphori in different degrees of 

 light, and found them imbibe most light from the sun itself, next in quantity, 

 when the sky was clear, and the least in foggy weather. These experiments 

 should be made in the open air, and not in a house with the glass windows shut; 

 because many bodies appear luminous, when the light has come directly to them, 

 which will not have that appearance when the light has passed through the glass. 

 He lastly tried what light they would imbibe from very bright flame, and found 

 that alabaster itself, which is saturated more than any substance by the sun's 

 rays, imbibed exceedingly little. The next query is, how long these bodies 

 should remain in the light to be sufficiently saturated; 4 or 5 seconds were found 

 the utmost length of time required for that purpose. The other query is, how 

 long the received light will continue in these phosphori? it does not last the 

 same time in all ; but continues more or less, from 2 seconds to 8, in proportion 

 to the strength of the phosphorus, and the quantity of light received. 



We pass now to the phosphori which are produced by art: -and first to those 

 which are made by the maceration of plants alone, and without any fire; such 

 as thread, linen cloth, but especially paper. The luminous appearance of this 

 last is greatly increased by heat. This is confirmed by two experiments: the 

 first is, by exposing the paper, spread on an iron grate, to the naked fire, yet 

 not near enough to scorch it, and then laying a warm brick on it to retain the 

 heat ; by which means it was observed, that where the paper was not skreened by 

 the iron grate it was most luminous; so that, by the lights and shades, might 

 be distinguished in the dark the image of the iron grate a considerable time. 

 The other experiment is the application of the paper to a plate of warm brass; 

 from which, when in the dark, you might very easily, by its being less luminous, 

 distinguish the margin of the paper, that had not been warmed by the brass. 

 The author proceeds to take notice of those phosphori which become so by 



