March 27, 1884] 



NA TURE 



517 



the focus of a system of lenses, so that the rays may cross in the 

 interior of the prism. This is an unfavourable position for a 

 prismatic analyser, and in the case of a powerful beam of light, 

 such as that from the electric arc, the crossing of the rays within 

 the prism is not unattended with danger to the cementing 

 substance, and to the surfaces in contact with it. 



Philip R. Sleeman 



ON VARIOUS SUGGESTIONS AS TO THE 

 SOURCE OF ATMOSPHERIC ELECTRICITY^ 



■\XTE have seen that, taking for granted the electrification of 

 clouds, all the ordinary phenomena of a thunderstorm 

 (except ^/(;fc lightning) admit of easy and direct explanation by 

 the known laws of statical electricity. Thus far we are on 

 comparatively sure ground. 



But the case is very different when we attempt to look a little 

 farther into the matter, and to seek the source of atmospheric 

 electricity. One cause of the difficulty is easily seen. It is the 

 scale on which meteorological phenomena usually occur ; so 

 enormously greater than that of any possible laboratory arrange- 

 ment that effect-:, which may pass wholly unnoticed by the most 

 acute experimenter, may in nature rise to paramount importance. 

 I shall content myself with one simple but striking instance. 



Few people think of the immense transformations of energy 

 which accompany an ordinary shower. But a very easy calcula- 

 tion leads us to startling results. To raise a single pound of 

 water, in the form of vapour, from the sea or from moist ground, 

 requires an amount of work equal to that of a horse for about 

 half an hour ! This is given out again, in the form of heat, by 

 the vapour when it condenses ; and the pound of water, falling 

 as rain, would cover a square foot of ground to the depth of 

 rather less than one-fifth of an inch. Thus a fifth of an inch of 

 r.iin represents a horse-power for half an hour on every square 

 foot ; or, on a square mile, about a million horse-power for four- 

 teen hours ! A million horses would barely have standing room 

 on a square mile. Considerations like this show that we can 

 account for the most violent hurricanes by the energy set free by 

 the mere condensation of vapour required for the concomitant 

 rain. 



Now the modem kinetic theory of gases shows that the par- 

 ticles of water-vapour are so small that there are somewhere about 

 three hundred millions of millions of millions of them in a single 

 cubic inch of saturated steam at ordinary atmospheric pressure. 

 This corresponds to 1/1600 or so of a cubic inch of water, i.e. to 

 about an average raindrop. But if each of the vapour particles 

 had been by any cause electrified to one and the same potential, 

 and all could be made to unite, the potential of the raindrop 

 formed from them would be fifty million million times greater. 



Thus it appears that if there be any cause which would give 

 each particle of vapour an electric potential, even if that potential 

 were far smaller than any that can be indicated by our most 

 delicate electrometers, the aggregation of these particles into 

 raindrops would easily explain the charge of the most formidable 

 thundercloud. Many years ago it occurred to me ihat the mere 

 con/act of the particles of vapour with th ise of aii-, as they inter- 

 diffuse according to the kinetic theory of gases, would suffice to 

 produce the excessively small potential requisite. Thus the 

 source of atmospheric electricity would be the same as that of 

 Volta's electrification of dry metals by contact. My experiments 

 were all made on a small scale, W:th ordinary laboratory appa- 

 ratus. Their general object was, by various pi ocesses, to pre- 

 cipitate vapour from damp air, and to study either (i) the 

 electrificati )n produced in the body on which the vapour was 

 precipitated ; or (2) to find on which of two parallel, polished 

 plates, oppositely electrified and artificially cooled, the more 

 rapid deposition of mois ure would take place. After many 

 trials, some resultless, others of a more promising character, I 

 saw that experiments on a comparatively large scale would be 

 absolutely necessary in order that a definite answer might be 

 obtained. I communicated my views to the Royal Society of 

 Edinburgh in 1S75, '" order that some one with the requisite 

 facilities might be induced to take up the inquii7,, but I am not 

 aware that this has been done. 



I may briefly mention some of the more prominent attempts 

 which have been made to solve this curious and important 

 problem. Some of them are ludicrous enough, but their diversity 

 well illustrates the nature and amoun- of the difficulty. 



■ By Prof. Talt. Read at the meeting of the Scottish Meteorological 

 Society on March 17, and communicated by the Society. 



The oldest notion seems to have been that the source of atmo- 

 spheric electricity is aerial friction. Unfortunately fur this 

 theory, it is nol usually in windy weather that the greatest deve- 

 lopment of electricity takes place. 



In the earlier years of this century Pouillet claimed to have 

 established by experiment that in all cases of combustion or 

 oxidation, in the growth of plants, and in evaporation of sail 

 water, electricity was invariably developed. But more recent 

 experiments have thrown doubt on the first tno conclu-ions, and 

 have shown that the third is true only when the salt water is 

 boiling, and that the electricity then produced is due to friction, 

 not to evaporation. Thus Faraday traced the action of Arm- 

 strong's hydro-electric machine to friction of the steam against the 

 orifice by which it escaped. 



Saussure and others attributed the production of atmospheric 

 electricity to the condensation of vapour, the reverse of one of 

 Pouillet's hypotheses. This, however, is a much less plausible 

 guess than that of Pouillet ; for we could understand a particle 

 of vapour carrying positive electricity witli it, and leaving an 

 equal charge of negative electricity in the water from which it 

 escaped. But to account for the separation of the two electrici- 

 ties when two particles of vapour unite is a much less promising 

 (ask. 



Peltier {followed by Lamont) assumed that the earth itself has 

 a permanent charge of negative electricity whose distribution 

 varies from time to time, and from place to place. Air, 

 according to this hypothesis, can neither hold nor conduct elec- 

 tricity, but a cloud can do both ; and the cloud is electrified by 

 conduction if it touch the earth, by induction if it do not. But 

 here the difficulty is only thrown back one step. How are we 

 to account for the earth's permanent charge ? 



Sir W. Thomson starts from the experimental fact that the 

 layer of air near the ground is often found to be strongly electri- 

 fied, and accounts for atmo-pheric electricity by the carrying up 

 of this layer by convection currents. But this process also only 

 shifts the difficulty. 



A wild theory has in recent times been proposed by Becquerel. 

 Corpuscles of some kind, electrified Ijy the outbursts of glowing 

 hydrogen, travel from the sun to the upper strata of the earth's 

 atmosphere. 



Miihry traces the source of electricity to a direct effect of 

 solar radiation falling on the earth's surface. 



Liiddens has recently attributed it to the friction of aqueous 

 vapour against dry air. Some still more recent assumptions 

 attribute it to capillary surface-tension of water, to the produc- 

 tion of hail, &c. 



Blake, Kalischer, &c., have lately endeavoured to show by 

 experiment that it is not due to evaporation, or to condensation 

 of water. Their experiments, however, have all been made on 

 too small a scale to insure certain results. What I have just 

 said about ihe extraordinary number of vapour particles ina 

 single raindrop, shows that the whole charge in a few cubic 

 feet of moist air may altogether escape detection. 



And so the matter will probably stand, until means are found 

 of making these delicate experiments in the only way in which 

 success is likely to be obtained, viz. on a scale far larger than is 

 at the command of any ordinary private purse. It is a question 

 of real importance, not only for pure science but for the people, 

 and ought to be thoroughly sifted by means which only a wealthy 

 nation can provide. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE 

 Cambridge. — The General Board of Studies propose to 

 appoint, early in Easter Term, a number of Readers and Uni- 

 versity Lecturers, including the following : a Reader in Com- 

 parative Philology, stipend 300/. per annum ; a Reader in 

 Botany, stipend 100/. ; University Lecturers in Sanskrit, in 

 Comparative Philology, in Mathematics (one in each group of 

 the Tripos, Part 3), in Applied Mechanics, in Botany, in Ani- 

 mal Morphology, in Advanced Physiology (three), in Geology, 

 in History (five), and in Moral Science ; all at 50/., except in 

 Animal Morp'nology and in Geology, to which 100/. is assigned. 

 The University Lecturers will for the most part be chosen from 

 such College Lecturers as open their lectures to the University 

 ge .erally ; but the Board is not necessarily restricted to such ; 

 nor to persons who may apply. Candidates are to send in their 

 names and testimonials (if any) to the Vice-Chancellor not later 

 than April 25. It is understood that two lectures a week during 



