April 26, 1906] 



LETTERS TO THE EDITOR 



NA TURE 



607 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



Diurnal Variation of Ionisation in Closed Vessels. 



The connection between the periodicity in the ionisation 

 in closed vessels and the variation in the intensity of the 

 electric field near the earth's surface suggested by Messrs. 

 Campbell and Wood in Nature of April 19 (p. 583) may 

 readily be explained on the theory of conduction through 

 gases if we assume that the ionisation is caused by radi- 

 ation from extra-terrestrial sources. 



The view of the origin of the earth's field which appears 

 to be in closest agreement with the facts is that it arises 

 in rainy regions on account of the negatively charged rain 

 conveying its charge to the earth, which thus becomes 

 negatively charged. This leaves a high positive potential 

 in the atmosphere immediately above the rainy region 

 which very rapidly distributes itself over the earth's sur- 

 face by means of discharges in the upper regions of the 

 atmosphere where the pressure is low enough for ionisation 

 by collisions to occur. Owing to the high conductivity of 

 the upper regions of the atmosphere, therefore, the potential 

 will differ only to a relatively slight extent over different 

 regions of the earth's surface; most of the fall of potential 

 between the positive charge over the rainy region and any 

 point of the earth's surface will occur in the badly conduct- 

 ing layer of air at a high pressure, which is comparatively 

 close to the earth's surface. 



The above theory is due to Mr. C. T. R. Wilson, with 

 whom I have recently discussed the matter. The explan- 

 ation of the connection between the earth's field and the 

 ionisation in closed vessels which follows might be made 

 to fit other views of the nature of the earth's field, but I 

 have selected Mr. Wilson's, as it appears to be the most 

 probable. 



The distribution of the earth's field, then, reduces itself 

 to a case very similar to that between two plane electrodes 

 immersed in a gas and maintained at a constant difference 

 of potential. Consider what happens if we increase the 

 ionisation near one electrode to a greater extent than that 

 near the other. The potential gradient will become smaller 

 where the ionisation is greatest, and conversely. In the 

 case of the earth the ionising ravs presumably come 

 from extra-terrestrial sources, and will be absorbed to some 

 extent by the earth's atmosphere. They will therefore be 

 more intense further away from the earth's surface, and 

 when for some reason or another they increase in intensity 

 they will increase the ionisation at a point some distance 

 from the earth's surface to a greater extent than at a point 

 near to it. From what has been said above, an increase in 

 the ionising rays should therefore produce an increase in 

 the electric intensity close to the surface, and vice versa. 



This corresponds exactly with what Messrs. Campbell 

 and Wood have found to be the case ; the maxima and 

 minima in the earth's electric field are simultaneous re- 

 spectively with the maxima and minima in the ionisation 

 in a closed vessel. 



It may be of interest to add that Borgmann (]urn. 

 Russk. Fisik. Chimicesk. Obscestva [physical part], xxxvii., 

 No. 4, pp. 77—98, 1905) has also recorded a minimum at 

 about 3 p.m. in the ionisation in a closed vessel. The fact 

 that the time is nearly the same at Cambridge and St. 

 Petersburg seems to indicate that the ionisation is caused 

 by radiation coming from the sun. The fact that the daily 

 variations in the earth's field are conditioned by the sun 

 has already been remarked by meteorologists (cf. Arrhenius, 

 " Kosmische Phvsik," ii., p. S90). 



I wish to point out finally that the above explanation 

 of the changes in the earth's field does not depend essential! \ 

 on radiation coming from extra-terrestrial sources. Any 

 cause which simultaneously increased the penetrating 

 radiation near the earth and the ionisation further away 

 from it would work in the same wav. 



O. W. Richardson. 



Trinity College, Cambridge, April 22. 



NO. I904, VOL. J 3] 



The New Spot on Jupiter. 

 The recent outbreak of dark material in the north 

 equatorial belt and north tropical zone of Jupiter has 

 further intensified, and forms a very prominent and striking 

 feature in the region north-following the red spot. The 

 slanting belt, alluded to in my letter published in last 

 week's Nature (p. 584), appears to be rapidly extending 

 in a longitudinal direction, and the large dark oval spot 

 on its following side has been several til 

 here. Transits were obtained as under : — 



es re-observed 

 Longitude 



April 15 



75-2 

 77 5 

 777 



The rate of motion appears, therefore, to conform very 

 nearly with that of the red spot and of system ii. of 

 Crommelin's ephemerides (9I1. 55m. 40-6s.). On April 20 

 the north tropical spot was very distinctly seen nearly two 

 hours before sunset, and the transit obtained on that date 

 was regarded as accurate. 



The preceding side of the slant-belt is moving much 

 faster than the north tropical spot, and it is highly prob- 

 able that in a few weeks a new and conspicuous belt will 

 have formed and entwined itself completely round the 

 planet. In this phenomenon we have a repetition of that 

 observed in the spring of i860 (see Monthly Notices R.A.S., 

 April, i860, and December, 189S, vols, xx., p. 244, and 

 lix., p. 76). W. F. Denning. 



BristoL- April 2-;. 



}/ Utilisation of Nitrogen in Air by Plants. 



Your reviewer (p. 531) of the above work has, like others, 

 failed to furnish any proof against my theory of the fixation 

 of free nitrogen by' plants. He desiderates direct chemical 

 proof of the increase of nitrogen in the plant, beyond the 

 nitrogen that is provided by the seed and the soil. Those 

 acquainted with agricultural chemistry know the difficulty 

 of directly determining a slight increase in the quantity of 

 nitrogen, in the circumstance of the comparatively large 

 quantity of nitrogen in the soil necessary to produce a 

 vigorous plant, and they will understand how difficult it is 

 to produce such proof; with the greater information now 

 available, however, it may now be forthcoming. But for 

 this difficulty, the fixation of nitrogen would have been 

 found out long ago. 



The experiments at Rothamsted conducted by Lawes 

 and Gilbert are identified with the subject of nitrogen. 

 The idea of the inability of plants to fix free nitrogen is 

 largely based on their 'experiments. As mentioned in a 

 book written by the recently appointed director at Roth- 

 amsted— Mr. A. D. Hall— it occupied their minds " from 

 the verv beginning of their experiments until the end." 

 It was their "dominant idea." 1 may therefore refer to 

 experiments carried out there which show that Lawes and 

 Gilbert themselves found (as many others have done) an 

 increase in nitrogen in growing crops, the source of which 

 could only be ascribed to the atmosphere ; thus (see p. 10), 

 " As a result of three years' cropping with barley and 

 clover, and then with clover only, an average amount of 

 319-5 lb. of nitrogen was removed, yet the soil contained, 

 on analysis at the end of the experiment, 2832 lb. of 

 nitrogen per acre in the top 9 inches, or a gain of 175 lb 

 per acre in the three years, making a total, with the crop 

 removed, of nearly 500 lb. of nitrogen per acre to be 

 accounted for." This was a troublesome fact. It was 

 sought to be explained by the tubercles on legumes, but 

 that an increase was got without legumes is shown by 

 another set of experiments (see p. 8) : — " the various crops 

 were grown continuously with mineral manures, but with- 

 out any supply of combined nitrogen ; the following average 

 amounts of nitrogen per acre were taken away : — 



lb. 



of which two fallow 

 6 crops only 



221 



22 '4 

 1 6 4 

 45'5 

 39-8" 



