NATURE 



[October 27, 1904 



Expansions exceeding the limit, r,/7;, = i-3S, however, give 

 fogs, which increase rapidly in density, i.e. in the niimber 

 of the drops, as the expansion is increased beyond this second 

 limit. The expansions required for the rain-like and cloud- 

 lil<e -condensations correspond to a fourfold and eightfold 

 supersaturation respectively. 



A further experiment will throw light on the nature of 

 the nuclei associated with the rain-like condensation. Let 

 us expose the moist air to the action of X-rays before causing 

 it to expand. First let us try an expansion very slightly 

 less than that required to give the rain-like condensation 

 without the rays. You observe no drops are formed. Now- 

 let the expansion be slightly greater than the critical value 

 1-25, A fog is seen on expansion. Thus the X-rays pro- 

 duce in the air immense numbers of nuclei having the same 

 properties, so far as their power of assisting condensation 

 goes, as the comparatively few nuclei which the rain-like 

 condensation makes visible. Now a gas exposed to X-rays 

 conducts electricity, and the otherwise complicated pheno- 

 mena of this conduction are all reduced to comparative 

 simplicity by the theory that under the action of the rays 

 equal numbers of freely moving positively and negatively 

 electrified bodies (the ions) are produced from the originally 

 neutral gas. It is at once suggested that the condensation 

 nuclei produced by X-rays are simply these ions. 



Let us now impart conducting power to the gas by ex- 

 posing it to the action of the radiation from radium. Again 

 we have the same result ; no drops are produced if the 

 expansion be less than i 25, fog if the expansion exceeds 

 this limit. 



If we substitute for the glass shade, which has thus far 

 formed the cloud-chamber, a glass cylinder with a horizontal 

 metal top, we have the means of testing whether the con- 

 densation nuclei produced by Rontgen or radium rays are 

 really electrically charged, whether, in fact, it is the ions 

 themselves which act as condensation nuclei or other 

 particles produced by the rays. If, for example, the roof of 

 the cloud chamber be kept positively charged, the floor 

 negatively, the negatively charged ions will travel upwards 

 and the positively charged ones downwards. In the absence 

 of an electric field the positive and negative ions produced 

 by the action of the rays will go on increasing in number 

 until as many are neutralised by recombination with ions 

 of the opposite kind, or by coming in contact with the walls 

 of the vessel, in each second as are set free in that time by 

 the rays. If the rays be cut off, the removal of ions by 

 recombination and diffusion will continue, and the number 

 of ions in the vessel will diminish rapidly. 



Experiment shows that, while in the absence of an electric 

 field, quite a considerable fog is formed when an expansion, 

 slightly exceeding 125, is effected ten seconds after the rays 

 have been cut off, with 200 volts between the upper and 

 lower plates the same expansion, allowed to take place 

 three or four seconds after the stopping of the rays, produces 

 only a very slight shower. Or, again, if the rays be kept 

 on all the time the resulting fog is very much less dense 

 with the electric field acting than without it. These results 

 are easily explained if we assume that the condensation 

 nuclei are the ions, and apply the result obtained by purely 

 electrical methods, that the ions travel about i-6 cm. per 

 second in a field of i volt per cm. The nuclei causing the 

 rain-like condensation without exposure to Rontgen or 

 radium rays are also removed by the action of an electric 

 field ; we have thus the direct proof that they also are ions. 

 Recent experiments have proved that a charged conductor 

 suspended within a closed space loses its charge by leakage 

 through the air, and that the conduction shows all the 

 peculiarities of that met with in an ionised gas ; and, 

 indeed, it appears that this ionisation is due to the action 

 of radiation of the radium type from the walls of the vessel 

 and from outside the vessel. The condensation method of 

 detecting ions is, it may be pointed out, a very delicate one ; 

 a single ion if present in the vessel will be detected. 



The positive and negative ions are not alike in their 

 power of acting as condensation nuclei. In most of the 

 experiments shown to-night the negative ions alone have 

 in fact come into action. The positive require a consider- 

 ably greater expansion in order that water may condense 

 upon them. The final volume must for the positive ions 

 be about 131 times the initial instead of only 1-25, corre- 

 sponding to a sixfold instead of a fourfold supersaturation. 



NO. 1826, VOL. 70] 



To demonstrate the difference between the positive and 

 negative ions the same form of apparatus is used as in the 

 previous experiment. Instead, however, of a difference of 

 potential of 200 volts, only 2 or 3 volts are applied 

 between the plates ; and in this experiment only a thin 

 layer close to the lower plate is exposed to the action of the 

 rays. Under these conditions, if the upper plate is the 

 positive one, the negative ions will be attracted upwards 

 out of the ionised layer, and will occupy the greater part 

 of the volume of the vessel, while the positive ones will 

 have only a short distance to travel before reaching the 

 lower plate. If the rays be cut off before the expansion 

 is made it is easy to arrange the interval to he of such a 

 duration that all the positive ions have been removed, while 

 only a small fraction of the negative ions have reached 

 the upper plate before the expansion takes place. Thus 

 we can try the effect of expansion when the vessel is charged 

 with practically negative ions only. By reversing the 

 electrical field the action of positive ions, almost free from 

 negative ions, can be studied. When the expansion is 

 between i 25 and 1-31 a fog or a mere shower is obtained, 

 according as the direction of the field is such as to drive 

 negative or positive ions upward. 



The ions are by no means the only nuclei which can be 

 produced within moist air from whi \ the dust particles 

 have been removed. Among the mosi interesting of such 

 apparently uncharged nuclei are those - iduced in moist air 

 exposed to ultra-violet light. It is in,. ,,ssible in the time 

 available to do more than allude to the.ii here. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Cambridge. — The reader in animal morphology (Mr. 

 Sedgwick) gives notice that a special course of advanced 

 lectures on certain general aspects of zoology will be given 

 at the zoological laboratory during the Slichaelmas and 

 Lent terms, beginning Friday, October 28. The course 

 will include lectures by the following, and will be given as 

 nearly as possible in the order indicated : — Michaelmas 

 term : Mr. Doncaster, the nucleus and heredity ; Mr. Lister, 

 Foraminifera and Mycetozoa ; Mr. Punnett, metamerism. 

 Lent term : Mr. Gardiner, the oecology of aquatic animals ; 

 Mr. Brindley, certain aspects of regeneration ; Mr. Hopkins, 

 animal pigments ; Mr. Fletcher, cell-structure, cell-division, 

 and maturation of germ-cells ; Mr. Heape, some problems 

 connected with the comparative physiology of the generative 

 system. 



Dr. Donald Mac.\lister, St. John's, who has represented 

 the university on the General Medical Council since 1889, 

 and is now chairman of the British Pharmacopoeia Com- 

 mittee, was re-elected for a fourth period of five years on 

 October 24. 



.■\ university lectureship in applied mathematics is vacant 

 by the appointment of Mr. H. M. Macdonald to be professor 

 of mathematics in the University of Aberdeen. The reader- 

 ship in botany is vacant by the resignation of Mr. Francis 

 Darwin. These offices will be filled up during the present 

 term. 



The Gedge prize in physiology has been awarded to Mr. 

 K. Lucas, fellow of Trinity, for his paper on " The 

 Augmentor and Depressor Effect of Tensions on the Activity 

 of Skeletal Muscle." 



The number of students of the first year matriculated on 

 October 21 was 884, or for the whole year up to that date 

 923- 



The late Mr. Henry Evans, of Trinity College, be- 

 queathed to the university his collection of British Lepi- 

 doptera. 



The following examiners have been appointed for the 

 natural sciences tripos : — Physics, R. T. Glazebrook and 

 W. C. D. Whetham : chemistry, H. O. Jones and Prof. 

 A. Smithells ; mineralogy. Prof. W. J. Lewis and L. J. 

 Spencer ; geology, A. Harker and Dr. F. A. Bather ; botany, 

 A. C. Seward and H. Wager ; zoology, A. Sedgwick and 

 Prof. W. A. Herdman ; physiology, "W. M. Fletcher and 

 Prof. E. Waymouth Reid ; anatomy. Dr. E. Barclay Smith 

 and Prof. A. Robinson. 



