1 1)6 An Esiuy on a Proper ty in Light 



wliich may easily be explained in the increased density and elas- 

 ticity of the air. 



The prevailing opinion however, at present, respecting the au- 

 rora borealis, is, that it is an electric fluid, though from all I have 

 read upon the subject, the only resemblance it has to an electric 

 fluid is, that light is to be found in both phasnomena. We are 

 told that the use of this meteor is to furnish lightning to the tor- 

 rid zone; but in what manner it communicates with the reservoirs 

 of the electric fluid, the clouds, has not been explained. For my 

 own part, I think it a great deal more likely that the combustible 

 matter, or, if it be more philosophical, the electric fluid in a dor- 

 mant state, with which the clouds are loaded, is carried up from 

 the earth in exhalations ; and that when these exhalations are 

 condensed into clouds, and excited, perhaps, by a heated atmo- 

 sphere,they explode and go off in lightning ; and this, most likely, 

 is the reason why there is more lightning in the torrid than the 

 temperate zone, in the summer than the winter ; for people al- 

 ways prognosticate a thunderstorm when the air appears to be 

 unusually heated. But, whatever may be the cause of the light- 

 ning, the aurora borealis is by no means fitted to furnish a sup- 

 ply of electric matter to the torrid zone ; for, if the electric fluid 

 (lightning) expends itself in flame, the aurora borealis also goes 

 off in flame, and its whole stock of electric matter would be ex- 

 pended before it got to the torrid regions ; and if it does not ex- 

 pend itself in flame, then it can stand in no need of a supply. 



So far then I have cleared the way for the introduction of my 

 own hypothesis ; and if I can but satisfactorily account for the 

 crackling, I can imagine no other objection that will at all af- 

 fect it, because all the other phasnomena, upon my principle, 

 may be proved by analogy, and accounted for by the known laws 

 of nature. 



I do not know whether any one has ever yet attempted to ex- 

 plain the cause of the refracting power of the atmosphere ; but it 

 must be obvious to every one that its principle is totally different 

 to that of every other medium. In every other medium the an- 

 gle of refraction is always in proportion to the angle of incidence, 

 let that angle be what it will ; but in the atmosphere this is not 

 the case, its figure has nothing to do with it, and the degree of 

 refraction is always in proportion to the denr,ity of its particles- 

 Light was supposed by bir Isaac Newton, if not by all the philo- 

 Sf>phers, to partake of the nature of matter, and therefore should 

 be subject to the laws of attraction as well as all other matter. 

 The power of attraction is always in proportion to the velocity 

 of the body in motion ; and the reason why light is refracted more 

 in a dense atmosphere than in a rare one, is not because there is 



any 



