IM/ 



'•] 



KNOWLEDGE & SCIENTIFIC NEWS. 



377 



causes assigned, some rather ludicrous — e.g., that it resulted 

 from the light passing through a film of water as the sun dipped 

 below the horizon. On the whole, I think Admiral Maclear's 

 quotation from Professor Davis' work appears Ihe most pro- 

 bable solution. I have noticed, in a favourable display, that 

 the " flash " changes rapidly from green to blue and violet, 

 which bears out Professor Davis' theory — the red and yellow 

 parts of the spectrum being lost in the general brightness of 

 the sky. 



With respect to your editorial note, I would mention that 

 far the finest view of the " green flash " is to be obtained, 

 according to my experience, oft repeated, at sunrise, when 

 there has been no previous "glaring " of the eye to cause the 

 complementary colour-effect. The observation is more un- 

 certain owing to the difficulty of knowing ixactly wliei-i the 

 sun will appear. But a look at the azimuth and amplitude 

 tables enables one to set the sight of the standard compass 

 e-xactly on the point, as a guide to the eye. And the horizon 

 is more apt to be clear and sharp at sunrise than at sunset, 

 while the brilliance of the flash to the un-tired eye is magnifi- 

 cent. I remember once in the South Atlantic lying in my 

 bunk watching a barque on the horizon, silhouetted black 

 against the eastern sky. and framed in the port-hole, when the 

 sun rose exactly behind it. and thesight was simply too beauti- 

 ful for words, from the play of bright blue and green light 

 among the tracery of masts and rigging. I have been the flash 

 twice at the same sunset, as we rose and fell on mighty rollers 

 in the Southern Ocean off Cape Leeuwin, of which I sent an 

 account to S'aturc. Some time in the late summer of iSgi I 

 was always careful in watching a likely sunset never to look 

 directly at the sun till the last moment, to avoid fatiguing the 

 eye with glare. Also, the best shows were when the sun, on 

 the horizon, was much distorted by refraction. 



C. MOSTVN. 



Tunbridge Wells, February 17, igo6. 



Mr. F. W. Levander, Editor of the Journal of the British 

 Astronomical Association, informs us that several communica- 

 tions on the phenomena referred to above are to be found in 

 vols. 7, 8, 10, II, 12, and 15 of that Journal. 



Electric Induction Experiments. 



Di-;ak Sir, — My attention has been called to an article 

 which appeared in your journal for August last, on "Some 

 Electrical Influence Experiments," by Mr. Charles E. Benham. 

 The conclusions arrived at in that paper seemed to be so 

 extraordinary, and so opposed to scientific principles, that I 

 have taken the trouble to repeat Mr. Benham's experiments. 

 Preparing three sheets of glass similar to those shown by Mr. 

 Benham in Figs, i and 2, page ig6, I repeated the experiment 

 as described in your journal, and got the same results as Mr. 

 Benham. But if his interpretation be correct, namely, that 

 the electrification is not due to friction, we would have all the 

 conditions necessary for producing perpetual motion, as the 

 energy necessary for making the successive contacts would 

 be less than might be developed by the electricity. For this 

 reason the presumption was there was an error somewhere. 



To find out what was taking place, I connected by means 

 of a fine wire the upper tinfoil F (see Benham's figures) with 

 a gold leaf electroscope, the plates being in position. The 

 strips of tinfoil on the lower plate were now successively 

 earthed, then the upper one was earthed, the lower ones 

 being again earthed in succession, then the top one, and so on 

 a great number of times. The upper plate was now raised, 

 but it showed no signs of electrification ; a result one would 

 naturally expect. Instead of now simply earthing the tinfoils 

 on the lower plate by simple contact without friction, they 

 were earthed as described by Mr. Benham, by drawing the 

 finger over the lower surface of the plate so as to earth them 

 in succession as before. When this was done the electroscope 

 showed signs of divergence, which fell on the upper plate 

 being touched, and again rose on stroking the under plate, 

 showing that the earthing of the lower tinfoils by stroking 

 gave rise to a disturbance of the electrification. ^\'h('n this 

 process was repeated a number of times the plates acquired a 

 considerable electric charge. If after the plates have become 

 charged in this manner we simply touch the lower plates in 

 succession, and then the upper one without friction, and do it 



a number of times, the plates gradually get discharged, as one 

 would expect, the process being a step-down discharge. 



With regard to Mr. Benham's statement that the electric 

 charge cannot be due to friction, because after the manipula- 

 tions described by him the plates are sometimes positive and 

 sometimes negative. On putting this point to the test, I find 

 that shellac varnish on glass when rubbed with the hand is 

 sometimes electrified + and sometimes — , which explains the 

 change of electrification in Mr. Benham's experiment. At 

 first this change in the electrification seemed to be 

 capricious, and it was hard to say what electrification it 

 would take ; but at last it came out that the sign of the 

 electrification depended on a number of things. The varnish 

 used in these experiments was the ordinary commercial spirit 

 varnish. The plates were coated by pouring the varnish over 

 them, draining the surplus oft", and drying with heat. It was 

 found that one piece of glass so treated became positively 

 electrified when rubbed with the hand, while another piece 

 became negatively electrified, but the electrification of neither 

 of them was constant. In damp air what was previously + 

 became — , whilst the one that was — remamed so, but 

 became + when the air was very dry. If the skin of the 

 hand is dry it tends to give +, and if moist — . The tip 

 of the fingers may give -f, while the pad formed by closing 

 the thumb on the first finger gives — . A moist, newly- 

 washed skin gives — , and a dry one +. If the surface of the 

 varnish be smooth it tends to give +, while if rough to 

 give — . If in place of using the whole varnish, only the 

 clear upper part that forms after the varnish has been stand- 

 ing some time be used, it gives a very fine, smooth surface, 

 which seems to give -)~ in most conditions of humidity of the 

 air, but becomes — if a very little olive oil or other similar 

 substance be on the fingers. From the above, as might be 

 expected, it is no unusual thing for a piece of glass to give 

 electricity of one sign on first rubbing, and of the opposite 

 sign on the second rubbing, changmg about in a seemingly 

 capricious manner, but in reality in response to changes in 

 the rubber or surrounding air. Spirit varnish generally con- 

 tains two gums, and the dift'erent proportions of these on the 

 plate seem to determine whether it will tend to give + or — 

 electricity. J. A. 



The Electric Prodviction of Nitrates 

 from tKe Atmosphere. 



Abstract of a Di.Hourse delivered at the Royal Institution 



on Friday, February 2, 1906, by 



Professor .Sii.va.nus P. Thompson, D.Sc, F.R.S. 



.As the demand of the white races for wheat as a food- 

 stuff increases, the acreage devoted to wheat-growing 

 increases, but at a less rapid rate; and being limited by 

 climatic conditions will in a few years, perhaps less than 

 thirty, be entirely taken up Then, as Sir Wm. Crookes 

 ]3ointed out in his Presidential .Address in 1S98, there 

 will be a wheat famine, unless the world's yield j>er 

 acre (at present about 12.7 bushels per acre on the 

 average) can be raised by use of fertilisers. Ot such 

 fertilisers the chief is nitrate of soda, exported from the 

 nitre beds in Chili. The demand for this has ri.sen from 

 I, GOO, 000 tons in 1892 to 1,543,120 tons in 1905; and 

 the supply will at the present rate I)e exhausted in less 

 than fifty years. Then the only chance of averting 

 starvation lies, as Crookes pointed out, through the 

 laboratory. 



In 1781, Cavendish had observed that nitrogen, which 

 exists in illimitable quantities in the air, can be cau.sed 

 to enter into combination with oxygen, and later he 

 showed that nitrous fumes could be produced by passing 

 electric sparks through air. .Although this laboratory 

 experiment had undoubtedly pointed the way, though 

 the chemistry of the arc flame had been investigated in 

 iSSo by Dcwar, and though Crookes and Lord Rayleigh 



