11 4 Chemisfry. [Lecture 28, 



when dried slowly. The sun has a remarkable 

 power in discharging colours, and a particular 

 effect on some saline bodies. The force of the 

 sun's rays is greatly increased by condensing 

 them, as by lenses, speculums, or burning- 

 glasses. Concave mirrors and refracting lenses 

 have been used to melt metals and stones that 

 were formerly thought fixed. The speculum 

 made by Viletti at Lyons was three feet in dia- 

 meter; its focus half an inch. The principal 

 point in the improvement of lenses is to form 

 the focus at as small a distance as possible ; but 

 this requires so great a convexity, that by the 

 thickness of the glass a great part df the light is 

 turned out of its course. Another kind of burn- 

 ing mirror is made of pasteboard, with gold leaf 

 burnished on it. Buffon had one of this kind, and 

 he describes its effects as astonishingly great. 



VII. Fuel, which is the most successful means 

 of procuring heat, for it affords it either gentle 

 or intense. Its varieties may be reduced to the 

 following heads : 



1. Fluid combustibles are the most equable 

 and manageable sources of heat; they burn by a 

 wick, as spirit of wine, oil, &c. But spirit of 

 wine is the best, since its vapour is freest from 

 soot, and we may increase or diminish the heat 

 by the number of wicks : the cotton is not con- 

 sumed in the least, as the heat sufficient to con- 

 vert the spirit of wine into the vapour which feeds 

 the flame is not capable of scorching vegetable 

 substances: spirit of wine (when highly rectified) 



