August 20, 1903] 



NATURE 



367 



is to be expected that it would be "set into powerful vibra- 

 tion and gi«7e its characteristic spectrum. 



In the experiments of Sir William Ramsay and Mr. 

 Soddy 30 milligrammes of radjum bromide, probably about 

 four months old, were used. If the o body is helium, the 

 amount of helium liberated by solution of the radium in 

 water must have been between 000017 and 00017 c.c, 

 assuming that all of the helium produced was occluded in 

 the mass of the substance. 



There is evidence of at least five distinct changes occur- 

 ring in radium, each of which is accompanied by the ex- 

 pulsion of an o particle. One of the products of these 

 changes is the radium emanation. It is of interest to 

 calculate the volume of the emanation occluded in radium 

 when in a state of radio-active equilibrium. Taking as the 

 simplest hypothesis that one a particle is projected at each 

 liange, the number of atoms of the emanation produced 

 ))t<r second is 1/5 of the number of o particles, i.e. 

 1-3x10-* c.c. When radio-active equilibrium is reached, it 

 has been shown that 463,000 times the amount of emanation 

 produced per second is stored up in the radium. This 

 corresponds to 6x10-* c.c. The maximum amount of 

 emanation to be obtained from one gramme of radium 

 thus probably lies between 6x10-* c.c. and 6x10-* c.c. 



The radium emanation is the active principle of radium, 

 for about \ of the activity of radium is due to it. Thus 

 a large proportion of the radiations from radium is a direct 

 result of the changes occurring in the very minute amount 

 of matter constituting the radium emanation. If ever i c.c. 

 of the radium emanation can be collected at one spot, it 

 will exhibit some remarkable properties. The powerful 

 radiations from it would heat to a red heat, if they would 

 not melt down, the glass tube which contains it. This 

 very rapid emission of energy, in comparison with the 

 amount of matter producing it, would continue for several 

 flays without much change, and would be appreciable after 

 a month's interval. The very penetrating rays from it 

 would light up an X-ray screen brilliantly through a foot 

 of solid iron. E. Rutherford. 



Hpttws-y-Coed, August 15. 



Summer Lightning. 

 Although a good deal has been written on the subject 

 of "summer lightning," it may not be superfluous to 

 describe a display of the phenomenon which occurred here 

 last evening on a scale far surpassing anything which it 

 had been my good fortune to witness before. There had 

 been several thunderstorms in the district during the previous 

 fi%e or six days, and a few peals were heard and heavy rain 

 fell in the early afternoon of the day before (August 13). 

 Rut the sky cleared rapidly thereafter, and the evening and 

 night of that day were cloudless, every peak and crest 

 standing out sharply defined in the clear air. Yesterday 

 was still fine, but warmer and less bracing than visitors 

 here expect. Late in the afternoon wisps of white mist 

 began to gather round the summit of the Jungfrau, and 

 streaks of thin cloud took shape in the higher air above 

 the great mountain ridge that extends from the Silberhorn 

 to the Breithorn. About 8 p.m. I noticed a faint quivering 

 light overhead, supplemented by occasional flashes of 

 greater brilliance and different colour. These manifest- 

 iitions rapidly increased in distinctness, and continued to 

 [)iay only along the opposite mountain-ridge, not extend- 

 ing into the regions beyond, so far as these could be seen 

 from here, though I have since learnt that an independent 

 -.nries of flashes was seen around the Schillhorn on this 

 -ide of the valley. Not a single peal of thunder was at 

 any time audible. A long bank of cloud formed at a higher 

 level than the summits of the mountain-ridge, and at some 

 distance on the further side of it, so that the stars, else- 

 where brilliant, were hidden along the strip of sky above 

 the crest. 



NO. 1764, VOL. 68] 



As one watched the display it was easy to distinguish 

 more definitely the two kinds of discharge. One of them 

 took the form' of a faintly luminous reddish or pink light, 

 which shot with a tremulous streamer-like motion in hori- 

 zontal beams that proceeded apparently from left to right, 

 as if their starting point lay somewhere about the back 

 of the Jungfrau. These streamers so closely resembled 

 the aurora borcalis that, had they appeared alone, one 

 would have been inclined to wonder whether the " northern 

 lights " had not here made an incursion into more southern 

 latitudes. So feeble were they when they sped across the 

 clear sky that the stars were clearly visible through them. 

 Sometimes they quivered on the far side of the cloud, 

 lighting up its' edges and shooting beyond it across the 

 still unclouded blue. At other times they appeared on this 

 side of the cloud, and showed the dark outline of the 

 mountains in clear relief against the luminous background. 

 They so rapidlv succeeded each other that they might be 

 said' to be continuous, a faint pinkish luminosity seeniing 

 to remain always visible, though pulsating in rapid vibra- 

 tions of horizontal streamers. 



The brighter discharges were not only far more brilliant, 

 but much more momentary. They had a paie bluish-white 

 colour, and came and went with the rapidity of ordinary 

 lightning. But they were clearly connected with the 

 mountains, and not reflections from a series of distant 

 flashes. Sometimes they arose on the other side of the 

 great ridge, allowing its jagged crest to be seen against 

 the illuminated surface of the cloud beyond, but leaving all 

 the precipices and slopes on this side in shade. In ether 

 cases thev clearlv showed themselves on this side of the 

 mountains, lighting up especially the snow-basins and 

 glaciers with the dark crags around them. Nothing of 

 the nature of forked lightning was observed among them. 

 In one instance the flash or horizontal band of vivid light, 

 a mile or two in length, seemed to shoot upward from the 

 slope at the base of the precipices of the Silberhorn, as if 

 it sprang out of the ground, having a sharply defined and 

 brilliant base, rapidly diminishing in intensity upward, 

 and vanishing before reaching half-way up to the crest. 



But the most singular feature of the mere brilliant white 

 discharges was to be seen when one of the great couloirs 

 of snow or a portion of a glacier remained for a minute or 

 two continuously lu'ninous with a faint bluish-white light. 

 After an interval the same cr another portion, perhaps 

 several miles distant, would gleam out in the same way. 

 Mv first impression was that this radiance could only be a 

 reflection from some illuminated part of the cloud. But I 

 could not satisfy myself of the existence of any continuously 

 bright portions" of "the cloud. Moreover, the luminosity of 

 the snow and ice remained local and sporadic, as if the 

 beam of a search-light had been directed to one special 

 part of the mountain declivity, and then after a while to 

 another. While watching one of these patches of illumin- 

 ation, I noticed a bright point of light at the top of one 

 of the basins of neve on the slopes of the Mittaghorn. It 

 quickly vanished, but soon reappeared, and then as rapidly 

 was lost again. I thought that it was probably a star 

 briefly exposed through rifts in the cloud, though its posi- 

 tion seemed rather below that of the mountain-crest. Half 

 an hour later, however, a similar bright light appeared 

 about the same place, more diffused than the first, and 

 having a somewhat elongated shape. Whether it was really 

 a star seen through the distorting medium of a wreath 

 of mist, or a form of St. Elmo's fire clinging to some peak 

 on the precipice, could' not be ascertained from its 

 momentary visibility. 



I learnt this morning that other observers who could 

 watch at the same time the mountain ridjjes on each side 

 of the Lauterbrunnen valley noticed that sheet-lightning 

 was also playing about the Schilthorn, but quite indepen- 

 dently of that on the Jungfrau range, the one mountain 

 being dark, while the other was illuminated. The distance 

 of the two electric centres from each other is between five 

 and six miles. The whole display last evening afforded an 

 admirably complete demonstration of the erroneousness of 

 the notion formerly prevalent that summer lightning is 

 only the reflection of distant ordinary lightning, and of the 

 truth of the more recent views as to the nature of the 

 phenomenon. 



I may add that, as the lightning increased, the air, which 



