436 



NATURE 



\Scpt. 9, 1880 



of his information and his unwearied efforts to advance 

 zoological science. He was no less a thoughtful student 

 of the labours of others than a discover of new forms 

 and an accurate original inquirer. 



To one who had worked at the fauna of Siberia, at the 

 collections made during the Novara expedition and those 

 of the German exploring ship Gazelle, at the varied stores 

 in the " Museum Godeflroy " of Hamburg, who had made 

 himself familiar with the shores of the Adriatic and the 

 Mediterranean, as well as those of France and Britain, 

 the splendid zoological series made by H.M.S. Challenger, 

 under the direction of Sir Wyville Thomson and his 

 colleagues, could not but prove an irresistible attraction ; 

 and it was this which tempted him more than anything 

 else to make his last visit to this country in 1876, when 

 he attended the Meeting of the British Association in 

 Glasgow. 



Privately Prof. Grube was one of the most amiable and 

 accomplished of men. Of commanding presence (he was 

 a cuirassier in his youth), and frank and manly bearing, 

 his fund of general information, his musical tastes, and 

 great geniality, endeared him to all his friends. Nor wms 

 he less beloved as a teacher by his students. Full of life 

 and work, and with an industry that never seemed to flag, 

 he was suddenly cut off in the midst of his labours, and 

 just as he was organising fresh researches. 



A full biography of Prof. Grube will appear in the 

 Leopoldina in Halle, but, meanwhile, it is v/ell to 

 indicate in this country the sense of the great loss 

 which zoological science has sustained by the death of 

 this eminent investigator and teacher. W. C. M. 



THl 'SDERSTORMS 1 

 IV. 



ALMOST all the facts to which I have now adverted 

 point to water-substance, in some of its many forms, 

 as at least one of the chief agents in thunderstorms. And 

 when we think of other tremendous phenomena which 

 are undoubtedly due to water, we shall have the less 

 difficulty in believing it to be capable of producing 

 thunderstorms also. 



First of all let us think of some of the more obvious 

 physical consequences of a fall of a mere tenth of an inch 

 of rain. Suppose it to fall from the lowest mile of the 

 atmosphere. An inch of rain is 5 lb. of water per square 

 foot, and gives out on being condensed from vapour 

 approximately 3,000 units of heat on the centigrade scale. 

 The mass of the mile-high column of air a square foot in 

 section is about 3601b., and its specific heat about a 

 quarter. Thus its temperature throughout would be 

 raised by about 33° C. , or 60° F. For one-tenth inch of 

 rain, therefore, we should have a rise of temperature of 

 the lowest mile of the atmosphere amounting to 3*3° C., 

 quite enough to produce a very powerful ascending current. 

 As the air ascends and expands it cools, and more vapour 

 is precipitated, so that the ascending current is farther 

 accelerated. The heat developed over one square foot of 

 -the earth's surface under these conditions is equivalent to 

 work at the rate of a horse-power for twelve minutes. 

 Over a square mile this would be ten million horse-powgr 

 I"or half an hour. A fall of one-tenth of an inch of rain 

 over the whole of Britain gives heat equivalent to the 

 v.-ork of a million milhons of horses for half an hour ! 

 Numbers like these are altogether beyond the limits of 

 our understanding. They enable us, however, to see the 

 full explanation of the energy of the most violent hurri- 

 canes ir. the simplest physical concomitants of the mere 

 condensation of aqueous vapour. 



I have already told you that the source of atmospheric 

 electricity is as yet very uncertain. Yet it is so common 

 and so prominent a phenomenon in many of its mani- 



' Abstract of a lecture, delivered in the City Hall, Glasgow, by Prof. Tail. 

 Continued from p. 410. 



Testations that there can b6 little doubt that innumerable 

 attempts have been made to account for it. But when 

 we consult the best treatises on meteorology we find it 

 either evaded altogether or passed over with exceedingly 

 scant references to evaporation or to vegetation. Not 

 finding anything satisfactory in books, I have consulted 

 able physicists, and some of the ablest of meteorologists, 

 in all cases but one with the same negative result. 1 had, 

 in fact, the feeling which eveiy one must experience who 

 attempts to lecture on a somewhat unfamiliar subject, that 

 there might be much known about it which I had not 

 been fortunate enough to meet with. Some years ago I 

 was experimentally led to infer that mere contact of the 

 particles of aqueous vapour with those of air, as they fly 

 about and impinge according to the modern kinetic theory 

 of gases, produced a separation of the two electricities, 

 just as when zinc and copper arc brought into contact the 

 zinc becomes positively electrified and the copper nega- 

 tively. Thus the electrification was supposed to be the 

 result of chemical affinity. Let us suppose, then, that a 

 particle of vapour, after impact on a particle of air, 

 becomes electrified positively (I shall presently mention 

 experiments in support of this supposition), and see what 

 farther consequences will ensue when the vapour con- 

 denses. We do not know the mechanism of the precipi- 

 tation of vapour as cloud, and we know only partially that 

 of the agglomeration of cloud-particles into rain-drops ; 

 but of this we can be sure that, if the vapour-particles 

 were originally electrified to any finite potential, the 

 cloud-particles would be each at a potential enormously 

 higher, and the rain-drops considerably higher still. For, 

 as I have already told you, the potential of a free charged 

 sphere is proportional directlyto the quantity of electricity 

 on it and inversely to its radius ; so when eight equal and 

 equally charged spheres unite into one sphere of double 

 the radius, its potential is four times that of each of the 

 separate spheres. The potential in a large sphere, so 

 built up, is in fact directly proportional to its surface as 

 compared with that of any one of the smaller equal spheres 

 of which it is built. 



Now, the number of particles of vapour which go to the 

 formation of a single average rain-drop is expressed in 

 biUions of billions ; so that the potential of the drop 

 would be many thousands of billion times as great as that 

 of a particle of vapour. On the very lowest estimate this 

 would be incomparably greater than any potential we can 

 hope to produce by means of electrical machines. 



But this attempt at explanation of atmospheric elec- 

 tricity presents two formidable difficulties at the very 

 outset. 



1. How should the smaller cloud-particles ever unite if 

 they be charged to such high potentials, which of course 

 must produce intense repulsions between them ? 



2. Granting that, in spite of this, they do so unite, how 

 are they separated from the mass of negatively electrified 

 air in which they took their origin ? 



I think it is probable that the second objection is more 

 imaginary than real, since there is no doubt that the 

 diffusion of gases would speedily lead to a great spreading 

 about of the negatively electrified particles of air from 

 among the precipitated cloud-particles into the less highly 

 electrified air surrounding the cloud. And if the sur- 

 rounding air were equally electrified with that mixed with 

 the cloud, there would be no electric force preventing 

 gravity from doing its usual work. This objection, in 

 fact, holds only for the final separation of the whole 

 moisture from the oppositely electrified air ; and gravity 

 may be trusted to accomplish this. That gravity is an 

 efficient agent in this separation is the opinion of Prof. 

 Stokes. It must be observed that as soon as the charge 

 on each of the drops in a cloud rises sufficiently, the 

 electricity will pass by discharge to those which form the 

 bounding layer of the cloud. 



The first objection is at least partially met by the 



