1 64 



NA TURE 



[December 15, 1892 



compositions for the annual dues was also rejected. Dr. Allbutt 

 Regius Professor of Physic, has been appointed an Elector to 

 the Chair of Botany, in the place of the late Dr. Hort. 



The discussion on the plans for the new Geological Museum 

 (given at length in the University Reporter for' December 13) 

 was highly interesting, and appears on the whole to have been 

 favourable to the scheme proposed, subject to relatively unim- 

 portant modifications. Prof. Newton objected that the arrange- 

 ment of its contents should be zoological rather than strati- 

 graphical ; and the Registrary (Mr. J. W. Clark) took exception 

 to the plan of lighting, which would be better if it were from 

 the top rather than the sides. The geological staff were unani- 

 mous that the plan put forward was that which best met their 

 needs. It was agreed that the architectural effect of the museum 

 would be very fine, and worthy of Sedgwick's memory. 



SOCIETIES AND ACADEMIES. 

 London. 



Royal Society, November 24. — " Ionic Velocities." By 

 W. C. Dampier Whetham, B.A., Fellow of Trinity College, 

 Cambridge. Communicated by J. J. Thomson, F.R.S. 



From a series of determinations of the electrolytic conductivity 

 of various salt solutions combined with Hittorfs values for the 

 migration constants, Kohlrausch calculated the velocity of differ- 

 ent ions under a potential gradient of one volt per centimetre. 

 Dr. O. Lodge actually observed the velocity of the hydrogen 

 ion as it travelled along a tube filled with sodium chloride dis- 

 solved in jelly, decolourizing phenol-phthallein as it went. He 

 found -0029 cm. per sec, and Kohlrausch gives '0030. 



The author has measured the specific ionic velocity of other 

 ions by observing the motions of a junction between two salt 



solutions of slightly difl'eienl deisiiy and different colours, when 

 a current was passed across it. From the velocity of the bound- 

 ary, that of the ion causing the change in colour can be deduced, 

 The apparatus consisted of two vertical glass tubes about 2 cms. 

 in diameter, joined by a third considerably narrower, which was 

 bent parallel to the others for the greater part of its length. The 

 tube was filled with the solutions in such a manner that the 

 boundary was formed in the vertical part of the junction tube. 



When the solutions are of different specific resistances there 

 will be a discontinuity of potential gradient at the boundary 

 and a consequent electrification. The effect on the velocity of 

 the boundary is, however, non-reversible, and, for small differ- 

 ences, can be eliminated by taking the mean of the velocities in 

 opposite directions. The direct estimation of potential gradient 

 is unsatisfactory, but by measuring the current (7), the area of 



NO. 1207, VOL 47] 



cross-section of the junction tube (A), the specific resistance of 

 the solution (r), and the velocity of the boundary {v) we can 



find the specific ionic velocity v, for v, = ^ 



yr' 

 The first solutions used were those of copper and ammonium 

 chlorides dissolved in aqueous ammonia, the former being blue, 

 the latter colourless. The junction travelled with the current 

 with a velocity of i "57 cm. per hour going upwards and of i "60 

 cm. per hour coming downwards. The mean gives as the 

 specific ionic vel. of Cu in solutions of 'X gram, equiv. per 

 litre •000309 cm. per sec. This agrees exactly with Kohlrausch's 

 number for infinitely weak solutions of '00031 cm. per sec. 

 Other measurements were made for chlorine and for the 

 bichromate group (Cr207). 



The method was extended to alcohol solutions. The velo- 

 cities of both ions of a salt were determined by using two pairs of 

 solutions. Thus the velocity of chlorine was found by using a 

 cobalt chloride-cobalt nitrate pair, the colours of which are blue 

 and red respectively, and that of cobalt by a cobalt chloride- 

 calcium chloride pair, these being blue and colourless. The 

 sum of these velocities was compared with that deduced by 

 Kohlrausch's method from the conductivity of the solution. The 

 following are the results : — 



Specific Ionic Velocities. 

 I. — Aqueous Solutions. 

 Velocity 



Ion 

 Copper ... 



Chlorine ... 

 Bichromate 



observed 



/ 0*00026* \ 

 "t 0-000309 J 



o '0005 7 ) 

 000059* \ ••• • 



o '00048 i 

 group (Cr-P^)^ 0-00047 ^ ... . 

 0-00046 ) 

 Preliminary observations. 



Velocity 

 calculated 



0-00031 



o 00053 



0-000473 



Salt 

 Cobalt Chloride 

 Cobalt Nitrate 



Decembers.— 



II. — Alcoholic Solutions. 



Vel. of Anion Vel. of Ration Sum of vels. Sum ofvels. 

 observed observed observed calculated 

 0000026 0-000022 0000048 0000060 

 0-000035 0-000044 0-000079 0-000079 



On theVelocity of Crookes' Cathode Stream." 

 By Lord Kelvin, P.R.S. 



In connection with his splendid discovery of the cathode stream 

 (stream from the cathode in exhausted glass vessels subjected to 

 electric force), Crookes found that when the whole of the stream, 

 or a large part of the whole, is so directed as to fall on 2 or 3 

 sq. cm. of the containing vessel, this pirt of the glass becomes 

 rapidly heated up to many degrees, as much as 200° or 300" 

 sometimes, above the temperature of the surroundings. 



Let V be the velocity, in centimetres per second, of the 

 cathode stream, and p the quantity of matter of all the mole- 

 cules in I c.c. of it. Supposing what Crookes' experiments 

 seem to prove to be not far from the truth, that their impact on 

 the glass is like that of inelastic bodies, and that it spends all 

 their translational energy in heating the glass. The energy thus 

 spent, per square centimeire of surface struck, per second of 

 time, is ^pi/ ; of which the equivalent in gramme-water-centi- 

 grade thermal units is approximately ^^7^742,000,000. The 

 initial rate at which this will warm the glass, in degrees centi- 

 grade per second, is 



^Pt. (I), 



10* X 42 . o-a 



where a denotes the specific heat of the glass, and a the thick- 

 ness of it at the place where the stream strikes it. 



The limiting temperature to which this will raise the glass 



1 X ^P^' (2) 



E 42,000,000 

 where K denotes the sum of the emissivities of the two surfaces 

 of the glass in the actual circumstances. 



It is probable that p differs considerably from the average 

 density of the residual air in the enclosure. Let us take, how- 

 ever, for a conceivably possible example, p — lo-s, which is 

 what the mean density of the enclosed air would be if the 

 vessel were exhausted to 8 X 10-^ of the ordinary atmospheric 

 density. 



