July 21, 1887] 



NATURE 



285 



regard to continuation classes and to eveninjj classes, this Bill is 

 worthy of the serious consideration of the House. I hope that 

 hon. members will not at this time of the session overload the 

 Hill with amendments. 



SCIENTIFIC SERIALS. 



Bulletin de la Societi des Natunilistcs de Moscou, 1886, No. 3. 

 — On two great comets (41 and 42) of 1886, by Th. 

 liredichin (in French). — On the Agromyza lateralis and its 

 metamorphoses, by Prof. Lindeman (in German). — On the iron- 

 bearing mud of Lipetzk, by E. Kislakovsky. It appears much 

 like that of Franzensbad in Germany, and especially that of 

 Ciechocinek in Poland. — On the Ammonites of the group 

 Olcostephaniis versicolor (Trautschold), by Mary Pavlow (in 

 French, with two plates). Studying a rich collection of Ammo- 

 nites versicolor, some ofj which reach 8 inches in diameter, 

 while others have the size of a pin's head, the author considers 

 them as belonging to the genus Olcostephanus, and establishes 

 the following species, of which the last three are new : 0. 

 versicolor, elatus, subinversus, inversus, and coronatiformis, — 

 On the importance of oxygen for plants, by W. Palladin 

 (summed up in German). An elaborate research into the amount 

 of matter destroyed in consequence of fermentation in an atmo- 

 sphere devoid of oxygen, as also into the relations between 

 the breathing of plants and their growth. — On the dynamic 

 centra of a rotation-ellipsoid, with relation to earth, by K. 

 Weihrauch, being a mathematical inquiry (in German) from 

 which it results that the centres of attraction are situated ) the 

 earth nearer to the centre of figure than would be the case in 

 an homogeneous ellipsoid of the same average density. — On the 

 AlgJE of Moscow, by A. Artari (in French), being a continua- 

 tion of a former publication, and containing a list of eighty-five 

 more species, chiefly Bacillariaceae. — On the fauna of the lakes 

 of the Slavyansk mineral waters, by P. Stepanoff. The fauna 

 is mixed and contains representatives both of fresh-water and 

 marine species, these latter being chiefly found amidst the Infu- 

 soria;. — The » annual report of the Society contains obituary 

 notices of the late President of the Society, Dr. Renard. 



No. 4. — Vascular plants of Caucasus, by M. Smirnoff. In this 

 second paper (in French) the nebulosity of different partsof Cauca- 

 sus is discussed, and data given. — Wild plants of the Government 

 of Tambof, by D. Litvinoff, continued. — The species of Thrips 

 living on corn in Middle Russia, by Prof. Lindeman (in 

 German). The new species Thrips secalina and Phlocothrips 

 armata are described together with former ones. — Zoological 

 researches in the Kirghiz Steppe, by P. Nazarow, being a most 

 valuable review of the fauna of the steppe, especially of its avi- 

 fauna (with a map). — Speeches pronounced at the death of Dr. 

 Renard. 



SOCIETIES AND ACADEMIES. 

 London. 



Royal Society, June 16. — "Experiments on the Discharge 

 of Electricity through Gases." (Second Paper.) By Arthur 

 Schuster, F.R.S. 



In thinking over the phenomena presented to us in vacuum 

 tubes, I always felt a difficulty owing to our ignorance of the 

 conditions which hold at the surface of bodies, either suspended 

 in or near the discharge, or even at the boundary of the vessel 

 through which the discharge is passing. It is evident enough that 

 if there is a flow of electricity on the surface of a non-conductor 

 that flow must be tangential, but it is not so clear whether we 

 are justified to conclude from this that there can be no normal 

 forces at such surfaces, for it is not necessary that the flow should 

 always take place along the lines of force. 



Supposing we suspend two pieces of gold leaf, as in an elec- 

 troscope, at any place in a partially exhausted vessel, and render 

 them divergent by electrification, they should collapse as soon 

 as the discharge begins to pass, if tangential forces only can 

 permanently exist at their surface. This I have tested by experi- 

 ment, and found to be the case. 



A cylindrical glass vessel 38 centimetres high and 15 centi- 

 metres wide, was divided into two approximately equal compart- 

 ments by a vertical metallic screen. There was an open space 

 of about 5 millimetres between the screen and the sides of the 



vessel, a space of about 4 centimetres above, and 2*5 centimetres 

 below the screen. One compartment contained two pieces of 

 gold leaf, which could be charged from the outside. The other 

 compartment contained two electrodes about 5 centimetres apart, 

 and 2 centimetres from the screen ; these distances could be 

 varied during the experiment. The screen was always conducted 

 to earth, and the electric fields on the two sides of the screen 

 were therefore nearly independent of each other. When the 

 gold leaves were electrified and divergent, and discharges from 

 the induction-coil passed between the electrodes on the other 

 side, no effect could be observed at atmospheric pressure : the 

 gold leaves remained divergent. 



At a pressure of about 4'3 centimetres of mercury, the effect I 

 was looking for first appeared ; when the discharge passed, 

 the divergent leaves slowly collapsed, and as the pressure was 

 further diminished the collapse look place more and more 

 quickly. 



We have here, then, even with the discontinuous discharge, a 

 neutralization of all normal forces at the surface of the gold leaf. 



It seemed to me to be interesting to observe more particularly 

 the effects of the ordinary discharges we have at our command, 

 at atmospheric pressure. I took two light balls, and suspended 

 them so that they could be made to diverge by electrification. 

 The electrodes (either spheres or points) of a Voss machine were 

 placed at a distance of 3 inches from each other, and the electri- 

 fied balls were placed at a distance of 9 inches from the discharge. 

 The results are contained in the following table, in which the first 

 two columns indicate whether the electrodes of the Voss machine 

 were points or spheres. The third column gives the electrifica- 

 tion of the balls, and the fourth column the results. 



It will be seen that when the two electrodes are similar, 

 whether spheres or points, the balls collapse when they are 

 electrified positively only ; but that when one electrode is a 

 sphere and another a point, the balls collapse if their electrifica- 

 tion is of the opposite nature to that supplied by the point. 



The conclusion thus arrived at, which will be proved beyond 

 possibility of doubt in the second part of this paper, is this : we 

 can only have tangential forces at the surfaces of vessels inclos- 

 ing a gas through which a discharge is passing, provided no 

 current crosses the surface. 



After I had convinced myself that an electrified body placed 

 in a partial vacuum through which an electric current is going, 

 has its electricity quickly neutralized, it was doubtful still whether 

 this neutralization was due to an actual discharge or merely to a 

 covering of electrified particles of an opposite sign. The ques- 

 tion is a vital one in all cases where potentials have to be 

 measured. F'or we can only measure potentials of a gas by 

 measuring the potential of a metal in contact with it ; and if an 

 electrified body is covered by electrified particles of a different 

 sign, there is a finite difference of potential between the metal 

 and the gas, and we should have to inquire carefully, in each 

 particular case, how far such a difference would affect our 

 conclusions. 



The question is settled by the principal result of this paper : 



A steady current of electricity can be obtained in air from 

 electrodes at the ordinary temperature which are at a difference 

 of potential of one-quarter of a volt only (and probably less) ; 

 provided that an independent current is maintained in the same 

 closed vessel. 



In other words, a continuous discharge throws the whole vessel 

 into such a state that it will conduct for electromotive forces 

 which I believe to be indefinitely small, but which the sensitive- 

 ness of the galvanometer I used has prevented me from tracing 

 with certainty below a quarter of a volt. There cannot be there- 

 fore a finite difference of potential between a gas and a metal in 

 contact greater than that amount. 



The same vessel was used as in the previous experiment. 



