NATURAL PHILOSOPHY. 125 



aurora borealis, or australis, the height where this transit of electricity takes 

 place would be just that at which the density of the air is such as to render 

 it the best conductor. By careful measurement of the degree of attenuation 

 requisite to enable the electrical discharge to pass with the greatest facility 

 in our laboratory experiments, we may approximatively estimate the degree 

 of rarefaction of the atmosphere at the height where the aurora borealis 

 exists. By these means we get a mode of estimating the height of the au- 

 rora, by ascertaining, from the decrement of density in the atmosphere in 

 proportion to its distance from the earth, at what elevation the best conduct- 

 ing state, or that similar to our best conducting vacuum tubes, would be 

 found, or conversely, by ascertaining the height of the aurora, by parallac- 

 tic measurements, we may ascertain the ratio of decrement in the density of 

 the atmosphere. Thus, by our cabinet experiments, light may be thrown 

 on the grand phenomena of the universe, and the great questions of the 

 divisibility of matter, whether there is a limit to its expansibility, whether 

 there is a fourth state of attenuation beyond the recognized states of solid, 

 liquid, and gaseous, as Newton seemed to suspect (thirtieth query to the Op- 

 tics), and whether the imponderables are specific affections of matter in a 

 peculiar state, or of highly attenuated gaseous matter, may be elucidated. 

 Though the entire solution of such questions be beyond the power of man, 

 we may ever hope to gain approximative knowledge. The manageable 

 character of the electrical discharge, and the various phenomena it exhibits 

 when matter is subjected to its influence in all those varied states to which 

 we are enabled, by experiment, to reduce it, can hardly fail to afford new 

 and valuable information on these abstruse and most interesting inquiries. 



STATIC INDUCTION. 



The following is an abstract of a lecture recently delivered by Professor 

 Faraday, before the Royal Institution, on " Static Induction" : 



After referring to the simple case of evolution of electricity by the friction 

 of flannel and shellac, and tracing the effect upon their separation into ordi- 

 nary cases of induction, and after calling attention to induction as action at 

 a distance, and through the intervening matter, Professor Faraday pro- 

 ceeded to examine closely the means by which the state of the intervening 

 matter could be ascertained, choosing sulphur as the body, because of its 

 admirable nonconducting conditions, and its high specific inducting capacity. 

 It is almost impossible to take a block of sulphur out of paper, or from off 

 the table, without finding it electric; if, however, a small spirit-lamp flame 

 be moved for a moment before its surface, at about an inch distance, it will 

 discharge it perfectly. Being then laid on the cap of the electrometer, it 

 will probably not cause divergence of the gold leaves ; but the proof that it 

 is in no way excited is not quite secure until a piece of uninsulated tinfoil or 

 metal has been laid loosely on the upper surface. If there be any induction 

 across the sulphur, due to the feeble excitement of the surfaces by opposite 

 electricities, such a process will reveal it; a second application of the flame 

 will remove it entirely. When a plate of sulphur is excited on one side only, 

 its application to the electrometer does not tell at once which is the excited 

 side. With cither face upon the cap, the charge will be of the same kind; 

 but with the excited side downwards, the divergence will be much, and the 

 application of the uninsulated tinfoil to the top surface will cause a moder- 

 ate diminution, which will return as the tinfoil is removed; whereas, with 



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