84 M. Arago on Lightning. 



production of light. It is tliis oily matter, then, which takes 

 fire in the syringe, in consequence of the disengagement of the 

 heat which every strong compression of gas produces. And 

 hence we may remark, in passing, agreeably to the statement of 

 M. SaissT/ of Lyons, that the experiment does not succeed, ex- 

 cept with the assistance of the combustible gases themselves. 



The kind of lightning known by the name of forked-light- 

 ning, and in a zig-zag form, has always appeared so astonish, 

 ing, that individuals have been tempted to regard the appearance 

 as wholly illusory, and as the result of those irregulai- refrac- 

 tions to which the atmospheric vapours and the clouds might 

 subject luminous rays. (See Logan, in the Phil. Trans, vol, 39.) 

 But astronomers, who so frequently have occasion to examine 

 the heavenly luminaries through vapours and clouds, without 

 finding them more disturbed than if the atmosphere were se- 

 rene, cannot be induced even seriously to refute the strange 

 conception of Mr Logan. A flash of forked-lightning, in which 

 the zig-zag or angles are very acute, and still more one in whicli 

 there are two or three projecting points, presents so violent a 

 contrast to the regular curves which bodies subjected to the ac- 

 tion of accelerating forces undergo in their course, that we dare 

 not, at first glance, entertain the idea that such a flash exhibits 

 in the atmosphere the situations which one and the same matter 

 successively occupies. On the other hand, regard this light- 

 ning not as a material body, but an undulation, and the double, 

 triple, and other refractions which the luminous waves of cer- 

 tain crystals exhibit, become so many striking analogies on 

 which the mind can with complacency rest. We have only, 

 moreover, to recollect, that the atmosphere contains a great 

 variety of exhalations, and especially of aqueous vapours, very 

 irregularly distributed, whence it will result that it may oppose 

 a variety of unequal resistances to the progress of the lightning. 



The lightning, which appears in the form of balls, or fire- 

 balls, of which we have collected so many examples, afterwards 

 to be produced, and which are so extraordinary, first of all, by 

 the slowness and uncertainty of their motions, and then by the 

 extent of the devastation they occasion in bursting, appear to 

 me at present, among the most inexplicable phenomena in phy- 

 sics. These balls, or globes of fire, appear to be agglomera- 



