130 PHILOSOPHICAL TRANSACTIONS. [aNNO 1792. 



celain. Common earthen ware. Whinstone. Emery. Coal ashes. Sea sand. 



5. Gold, platina, silver, copper, iron, lead, tin, bismuth, cobalt, zinc^ 

 Precipitates by an alkali from acid solutions of gold, silver, copper, iron, zinc, 

 bismuth, tin, lead, cobalt, mercury, antimony, manganese. Vitriolated tartar, 

 crystals of tartar, borax, alum, previously exsiccated. Sea coal. White paper, 

 white linen, white woollen, in small pieces, white hair powder. Deal saw-dust. 

 Rotten wood (not otherwise luminous.) White asbestos. Red irony mica. 

 Deep red porcelain. 



6. Antimony, nickel. Oils, lamp, linseed, and olive, white wax, spermaceti, 

 butter, luminous at and below boiling. 



The duration of the light thus produced from different iaodies is very unequal ; 

 in some the light is almost momentary, in others it lasts for some minutes, and 

 may be prolonged by stirring the powder on the heater. It soon attains its 

 greatest brightness, and dies away gradually from that point, never appearing in 

 a sudden flash, like the light of quartz pebbles rubbed together. If blown on, 

 it is suddenly extinguished, but immediately re-appears on discontinuing the 

 blast. 



The light of bodies is, in general, uncoloured ; there are however some ex- 

 ce[)tions. Blue fluor, of that kind which gives out a fetid smell when rubbed, 

 first emits a bright green light, resembling that of the glow-worm so exactly, 

 that when placed by the insect just as it has attained its greatest brightness, there 

 is no sensible difference in the 2 lights, either of colour or intensity. This 

 bright green quickly changes into a beautiful lilac, which gradually fades away. 

 Fetid marbles, and some kinds of chalk, give a bright reddish or orange light ; 

 pure calcareous earth, a bluish white light; Cornish moorstone emits a fine blue 

 light ; powder of ruby gives a beautiful red light, of short continuance. 



Bodies are by far most luminous the first time they are heated, but cannot 

 perhaps be entirely deprived of this property by any number of heatings, nor by 

 any degree of heat. Chalk, fluor, and feldspat, give out a very faint light on 

 the heater, after having been exposed to a smart red heat in an open crucible, in 

 small quantities, and kept frequently stirred for several hours ; the feldspat was 

 equally luminous when laid hot on the heater, or first cooled, and then laid on. 

 Chalk and fluor were not tried in this particular. A bit of glass, melted in a 

 heat of 120° of his father's thermometer, and as soon as it is cold reduced to 

 powder, gives out light on being thrown on the heater below redness. Quartz 

 from the same original piece, is equally luminous when the powder is directly 

 thrown on the heater — when it is previously made red-hot, and then cooled and 

 thrown on — or when a fragment of some size has been made red-hot, then 

 pounded and thrown on. 



For the most part, the softest bodies require the least heat to become lumi- 



