32 



NATURE 



[March 13, 1913 



and distances measured above give positive values, and 

 those below negative. The extreme range of the 

 scale is equivalent to a gradient of 380 volts per metre. 



Fig. 2 gives the record for a dust-storm which lasted 

 the greater part of the day. In this figure it may be 

 seen that the positive value never goes beyond 70 

 volts per metre, and on one occasion even becomes 

 slightly negative. 



Fig. 3 records a severe dust-storm which lasted from 

 4 a.m. until 8 p.m. The maximum value of the 

 negative gradient cannot be inferred from the curve 

 because the electrometer needle was deflected as far as 

 it could go, and the horizontal portions of the curve 

 indicate that the potential gradient was higher than 

 the maximum which could be recorded. It will be 

 shown presently that the gradient may reach the 

 value of 5000 to 10,000 volts per metre when the dust 

 is blowing thickly. 



The writer has shown (Phil. Mag., May, 1912) that 

 during a dust-storm the charge upon the dust (if sili- 

 ceous) is positive, while that upon the air at the same 

 time is negative, and he was led from this to devise 

 an electrical machine by means of which charges of 

 both positive and negative electricity might be obtained 

 duiiiii; a dust-storm. The essential parts are : — (1) 

 A small insulated disc coated with radium attached 

 to a wooden rod about two metres in height ; (2) a 



Induced Cell-reproduction in the Protozoa. 



In the interesting letter by Mr. A. H. Drew, under 

 the above heading, in Nature, February 20, it is sug- 

 gested in the last paragraph that certain substances 

 called auxetics which caused the development of spores 

 in the case of new species of Polytoma, may be neces- 

 sary for cell-reproduction under natural conditions in 

 ponds, &c, where such substances would probably 

 occur owing to the putrefaction of organic matter. 



In the course of an investigation which I have 

 recently carried out on the process of excystation in 

 the ciliated infusorian, Colpoda cucullus, from its 

 resting cysts, I have found that this organism can 

 emerge from its cysts when the latter are incubated 

 in 1 per cent, hay infusion (alkaline or acid in reaction) 

 and in pure distilled water — media quite free from 

 auxetics. The real agent which is instrumental in 

 causing excystation is an enzyme which digests the 

 endocyst, and thus allows the organism to swim out 

 into the surrounding medium. As is well known, 

 Colpoda cucullus is an organism of wide distribution 

 and of common occurrence in ponds and in infusions 

 of hay, &c. It can frequently be found among rotting 

 grass and decaying vegetation ; situations in which 

 the products of organic decomposition and bacterial 

 putrefaction would be plentiful, yet the cysts of this 

 organism can be caused to rupture and yield their 



= (I) 



- (2) 



(1) Normal fine weather record (2) a n 



large hollow vessel with a fine wire gauze bottom ; 

 and (3) a pair of insulated spheres to serve as dis- 

 chargers. The hollow vessel generally used was a 

 five-gallon petrol tin supported upon an insulated rod 

 at a distance of about 20 cm. above the ground, and 

 directed with the open end towards the onrushing 

 dust. Much of this dust is carried through, but a 

 considerable portion is retained, and any charge it 

 may possess is given up to the vessel. This charge 

 was invariably positive. 



The radium-coated conductor, however, took the 

 negative potential of the current of air blowing past 

 it, so that the two balls acquired opposite charges, 

 and a torrent of sparks as continuous as that furnished 

 by an induction coil passed between them. On some 

 occasions the sparks reached a length of 1-5 cm., 

 showing that the potential difference between the 

 conductors must have been at least 40,000 volts when 

 the apparatus was set up on the open veld. 



An ordinary vacuum tube having a radium-tipped 

 wire attached to an electrode, the other electrode being 

 earthed, will light up brilliantly during the passage 

 of a dust-storm. A brush discharge is seen to proceed 

 from the electrode and the shape of the brush makes 

 it quite clear that positive electricity is escaping from 

 thi earth into the atmosphere. 



W. A. Douglas Rudge. 



NO. 2263. VOL. 91] 



Id dust-storm ; (3) a severe dust-storm. 



contents in active condition when incubated in pure 

 water. 



I would therefore suggest that it is unsafe to infer 

 that because auxetics may serve to induce cell-repro- 

 duction in certain cases, they may be necessary in all. 



The winter spores of Polytoma and the resting 

 (dauer) cysts of Colpoda are not perhaps quite com- 

 parable, but I may point out that Colpoda most fre- 

 quently encysts in the condition of the resting cyst, 

 and that therefore if auxetics are necessary at all 

 they ought to be required for excystation from this 

 condition. 



An account of my investigations on this subject will 

 shortly be published. T. Goodey. 



Rothamsted Experimental Station, Harpenden, 

 Herts, March 4. 



The Spectra of Neon, Hydrogen, and Helium. 



In a letter published in Nature of March 6, Prof. 

 Fowler pointed out that a series of "parallelisms" 

 that we gave of lines in the spectra of neon and 

 hydrogen were probably coincidences, and could not 

 be taken as evidence of identity. We are sorry that 

 we did not make our meaning plainer, in our letter 

 in Nature for February 27, for we did not mean that 

 the lines we compared in the two spectra were 



