270 



NA TURE 



[July 2 i, 1904 



trated extract. There is a capsule of chemical statics, 

 ■of dynamics, of physical mixtures, of thermochemistry, 

 of electrochemistry, &c. The same concentrated form 

 of diet is continued throughout the volume unrelieved 

 by any historical references or illustrations of 

 apparatus. 



There are numerous little inaccuracies, both of 

 author and printer, which it would be well to correct 

 in a future edition. J. B. C. 



LETTERS TO THE EDITOR. 



\[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 lo 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.] 



Residual Affinity. 



Sir Oliver Lodge and Prof. Frankland have indi- 

 •cated (pp. 176, 222) the way in which the electronic theory 

 may afford an explanation of various chemical phenomena ; 

 notably so in the case of solutions : the apparent dissociation 

 of the ions of the solute being a consequence of partial 

 withdrawal of the bonds or electric charges uniting them, 

 these bonds becoming occupied in connecting the ions 

 with the molecules of the solvent, and dissociation into ions 

 being thus a consequence of the chemical aflinity of the 

 dissolved substance for the solvent, instead of being a proof 

 that no such thing as chemical combination exists in a 

 solution. 



I should lilce to point out that this view was developed 

 by the writer nearly thirteen years ago in a paper entitled 

 *■ The Theory of Residual Affinity as an Explanation of the 

 Physical Nature of Solutions," which appeared in the 

 Berichtc, iSgi (pp. 3629-3447), and of which some account 

 will be found in the last edition of Watts 's " Dictionary of 

 Chemistry " under the head of " Solutions," p. 495. The 

 only difference in the explanation there given from that 

 given by Sir Oliver and Prof. Franl^Iand is that the atomic 

 charges were spoken of as fluid charges surrounding the 

 atoms instead of as Faraday bundles. 



The view that the charge uniting atoms in a molecule is 

 a variable quantity was developed by the writer at a still 

 earlier date in a paper on atomic valencv, read before 

 the Chemical Society, December 3, 1885, but printed pri- 

 vately only ; a further view was propounded in that com- 

 munication that the bonds or charge; of atoms of a different 

 nature were not exactly equivalent to each other, and were 

 not necessarily expressible by whole numbers. Such a view 

 gives a somewhat striking explanation of many chemical 

 facts which are otherwise difficult of explanation, but it is 

 independent of the explanation of the nature of solutions 

 given subsequently, and now put forward by Prof. Frank- 

 land, the basis of which is the mobility and divisibility of 

 the atomic charges. Spencer Pickering. 



Harpenden, July 10. 



A Volatile Product from Radium. 



In the course of some recent experiments on the excited 

 radio-activity from the radium emanation, some evidence 

 has been obtained which points to the conclusion that the 

 emanation X of radium at one stage of the changes which 

 it undergoes after being deposited on a solid bodv is slightly 

 volatile even at ordinary temperatures. The effect which 

 gives rise to this conclusion was first noticed in some 

 observations on the rate of decay of the part of the excited 

 activity deposited on a plate of copper immersed for a short 

 time in dilute hydrochloric acid, in which the activity from 

 a platinum wire exposed for a time to the radium emanation 

 had been dissolved. When the copper plate with its active 

 deposit had been placed inside a testing vessel and removed 

 after a few minutes, it was noticed that a temporary 

 activity, in some cases equal in amount to one or two per 

 cent, of the activity of the plate, was excited on the walls 

 of the vessel. This activity increased to about three times 

 its original value in the course of thirty minutes after the 



removal of the active copper, and then decayed regularly to 

 zero. 



The amount of this radio-active deposit that can be ob- 

 tained from a given amount of the direct radium excited 

 seems to be increased by the solution and re-deposit of the 

 emanation X, but it can also be observed from a wire just 

 removed from the radium emanation. If the active wire is 

 placed at once in the testing vessel without having had its 

 temperature raised in any way and removed in a few 

 minutes, an activity about i/iooo of the whole activity 

 shows itself on the walls of the vessel. The decay of the 

 activity of this deposit is the same as that of the deposit 

 obtained from the active copper. The following table gives 

 the rate of change of the radiation from the walls of a 

 vessel in which an active wire had been left for three 

 minutes after its removal from the emanation : — 



Time after removal ^ 

 in minutes ...j 

 Activity 40 61 75 96 99 100 



5 10 20 25 30 35 40 50 60 

 95 88 78 



NO. 18 I 2, VOL. 70] 



The active wire retains this power of exciting secondary 

 activity for only a short time after removal from the 

 emanation ; after ten minutes the amount it excites is 

 almost inappreciable. Merely washing the wire in a stream 

 of running water and drying it over a gas flame, as is 

 frequently done to prevent any trace of radium emanation 

 clinging to the wire, increases the amount of the secondary 

 activity to about 1/200 of the whole. 



It is evident, then, that some sort of volatile product is 

 given off from the active wire for a time which can excite 

 an activity the rate of decay of which would indicate two 

 changes in the active matter deposited, one producing rays 

 and the other not giving rise to any radiation (E. Ruther- 

 ford, "Radio-activity," p. 269). It is found that this 

 volatile substance responds to none of the three tests for an 

 emanation, it is not itself radio-active, it cannot pass with- 

 out sensible loss through material substances such as paper 

 and cotton-wool, and the activity due to it is not concen- 

 trated on the negative electrode in an electric field, but 

 distributes itself evenly over all surfaces exposed to it. 



The decav of the excited activity from the radium 

 emanation has been explained by Prof. Rutherford on the 

 assumption that there are three changes in the emanation 

 X after its deposit on a solid body. In these three stages 

 one-half the matter is changed in 3 minutes, 21 minutes, 

 and 28 minutes respectively. In the first and third stages 

 the change is accompanied by ionising rays, but the second 

 is a ravless change. Now if it be supposed that after the 

 first change has taken place the matter becomes slightly 

 volatile, and some of it is concentrated on surrounding 

 objects, a deposit would be obtained which would present 

 the tw3 remaining changes. From the equations for the 

 radio-activitv of such a deposit (" Radio-activity." p. 271), 

 it is found that the radiation would increase for about 34 

 minutes, pass through a maximum, and then decay at the 

 ordinary rate. This is very similar (o the behaviour of 

 the doDosit obtained in the .';bove experiments. 



Curie and Danne (Comptes rendns, March 21) have 

 obtained deposits showing similar characteristics by heating 

 a radio-active wire within a cylinder and measuring the 

 rate of decay of the activity of the cylinder. 



Harriet Brooks. 



McGill University, Montreal, June 28. 



The Traction of Carriages. 



It is a matter of general belief amongst drivers, owners, 

 and builders of carriages that if the distance between the 

 fore and hind wheels be increased so will the " draught " 

 be heavier. I have put the following case before a builder : 

 given two carriages weighing exactly the same, with the 

 fore and hind wheels of each of the same height, but the 

 body of one carriage much longer than that of the other, 

 then the horse will have as much to do in the one case 

 as in the other. The answer has been in more than one 

 instance, the longer bodied carriage will be the heaviest to 

 move. No reason has been given, nor can any explanation 

 of the existence of this belief be offered. Can any of the 

 readers of Nature make any suggestion ? 



Ross, July 17. E. Williams. 



