17 



between this and all other dissolvents in nature, that the 

 substance dissolved cannot unite with the dissolvent 

 without destroying its action. 



When salt dissolves in water, iron in aqua-fortis, or 

 gold in aqua-regia, the substance dissolved is equally 

 diffused through the dissolvent, so as to incorporate 

 with it; but non$ of the things dissolved by fire can 

 mix or incorporate with it : they all fly off in vapour, 

 otherwise the fire is presently extinguished. 



Elementary light then, the rays of which when con- 

 densed take the name of fire, is an element of a peculiar 

 kind, not subject to the mechanical laws of other bodies. 

 Now, if we suppose a material fluid, void of gravity, 

 pressure, or any other mechanical power, all gravitating 

 bodies will move through such a fluid, as freely as in 

 vacuo. 



Elementary light is a material fluid, void of gravity, 

 pressure, or any other mechanical power. When con- 

 deesed, it is pure elementary fire, which excludes all 

 other matter out of t lie same space : -yet it lies in the 

 focus of a burning-glass, perfectly still and quiescent. 

 Though it is surrounded by the air, which is a gravitating 

 fluid, pressing equally every way, yet this im mechanical 

 element is not at all affected by it, so as to rise or fall in 

 it, or in the least to alter its 'stale either of rest or mo- 

 tion, which must necessarily happen were it endued 

 with gravity, or the other mechanical properties found 

 in other bodies. 



And that the rays of light, in their progressive motion, 

 do not press, resist, attract, or at all disturb each other, is 

 evident from facts, though they come from every point 

 of space that can be within the optic angle of the eye. 

 Thus two men standing at a distance and looking at 

 each other, see one another at the fame instant and 

 that by means of rays, which act in contrary directions 

 without the least resistance. And any number of other 

 men, standing in any position, may see the same men in 



