October i8, 1906J 



NA TURE 



621 



vessel unless an electric field is present lo remove them as 

 they are set free; an expansion exceeding 125 gives, in 

 the absence of such a field, fog or rain, according as the 

 air is exposed to external ionising agents or not. The 

 above three limits would correspond to adiabatic pressure 

 falls of 27-7, 20-5, and 24-1 cm. of mercury respectively, if 

 ihe initial pressure was 76 cm., and would vary with the 

 initial pressure. The fog limit obtained by Prof. Barus 

 for air exposed to X-rays or radium rays, except under 

 conditions such that persistent nuclei resulted, generally 

 lay between ig and 21 cm., except when the radiation 

 was exceedingly weak, when the limit approached that 

 which he obtained for " non-energised " air, about 24 cm., 

 which may be compared with the intermediate critical 

 expansion mentioned above. The results of Prof. Barus 

 are accounted for if we suppose that his method failed to 

 detect the comparatively small number of drops formed 

 on the spontaneously produced negative ions ; such vari- 

 ation of the limit as was observed in air exposed to 

 external radiation, as the intensity was varied within 

 moderate limits, being what might be expected with a 

 method in which the " fog limit " is only reached when a 

 certain minimum number of drops is exceeded. It is true 

 that the ions are not at any one moment all in an equally 

 favourable condition for helping condensation, a certain 

 range of expansions (not very wide, however) being re- 

 quired, for example, to catch all the negative ions; but 

 there is no evidence that the efficiency of the ions as 

 nuclei increases with the intensity of the ionising rays, if 

 we leave out of consideration the possible effect of exceed- 

 ingly intense rays ; for the weakest radiation (that re- 

 sponsible for the " spontaneous " ionisation), as well as 

 for radiation of very considerable intensity, the efficiency 

 of the most favourably situated ions remains the same. 

 Prof. Barus has apparently failed to notice that the limits 

 found by him are, if properly interpreted, in fairly good 

 agreement with those of previous observers — quite as good 

 agreement as could be expected from the comparative 

 roughness of his methods. Possibly some explanation of 

 this omission is afforded by a passage on p. 50, where the 

 volume change corresponding to a given pressure fall has 

 been wrongly calculated, as if the expansion were iso- 

 thermal instead of being nearly adiabatic. 



It is a matter of some difficulty to know what views 

 Prof. Barus really holds upon the relation of the ionisation 

 as determined by electrometer measurements and the 

 " fieeting nuclei " which " most physicists would call 

 ions." That he does not regard such nuclei as identical 

 with the ions is plain from the statement that the gamma 

 rays, though weak ionisers, are strong nucleators, as well 

 as from the suggestion that the fleeting nuclei produced 

 by an X-ray bulb may be due to " a gamma-like ray," and 

 only the persistent nuclei to the " X-light properly so 

 called, which produces the well-known effects subject to 

 the law of inverse squares " (the ionisation as determined 

 by electrometer measurements being one of these, as 

 another of the passages quoted seems to indicate). Prof. 

 Barus seems to have entirely failed to realise how complete 

 Is the evidence of the identity of the nuclei produced, in 

 the investigations of previous observers, by X-rays or any 

 of the various types of Becquerel rays with the ions the 

 existence of which has to be postulated to explain the 

 phenomena of the conduction of electricity through the 

 air exposed to such rays. Not only has it been shown by 

 direct experiments that the nuclei are positively and neg.a- 

 tively charged bodies having properties such as have to be 

 assigned to the ions to explain the phenomena of conduc- 

 tion through gases, but a still more direct proof of the 

 identitv is furnished by the agreement of the two methods 

 bv which the charge on the ions was determined, that of 

 j! J. Thomson and that of H. k. Wilson. For the former 

 gives the ratio of the ionisation (the product of the number 

 of the ions per c.c. and the charge carried by each), as 

 determined by electrical methods, to the number of the 

 nuclei, while the latter gives directly the actual charge 

 of a single nucleus. Thus the number of nuclei, multiplied 

 bv the charge on each nucleus, is equal to the product of 

 ionic charge and number of ions deduced from electrical 

 measurements. The ionisation accounted for by the nuclei 

 in question is thus equal to the ionisation determined by 

 the electrical method. 



Chapters iv. and v. contain an account of observations 

 made at Providence and in the comparatively uncontamin- 

 ated atmosphere of Block Island upon the variations in the 

 number of nuclei in unfiltered atmospheric air. The nuclei 

 are here such as may be caught with smaller expansions 

 than are required by the ions; they are Aitken's "dust" 

 particles. Their number was estimated, not by .Aitken's 

 method, but by observing the coronas seen through the 

 fogs produced on expansion of the air in an apparatus of 

 the same type as that used in the investigations already 

 discussed. In the present case, where only easily caught 

 nuclei are involved, the objections brought above against 

 the method do not apply, and there can be no doubt about 

 the importance of such investigations. 



C. T. R. Wilson. 



BOTANICAL CONGRESS AT HAMBURG. 

 'T'HE Society of .Applied Botanists held its annual con- 

 ference at Hamburg in September, and the Societv 

 of Systematic Botanists held its meeting there at the same 

 time. Some 150 botanists in all, mostly interested in 

 applied botany, attended. The choice of place of meeting 

 was a happy one, as in Hamburg, the chief Continental 

 port, the closest connection can be seen between commercial 

 and scientific activities. 



All the botanical institutions are under the direction of 

 Prof. Zacharias, and while the educational requirements 

 are well cared for, everything that the botanical scientific 

 staff can do to foster the trade of the city is done. The 

 seed-testing station is under the direct charge of Prof. 

 Voigt, who, with six assistants, tests some 1500 samples 

 of seed, oil-cake, &:c., each year. An important export 

 seed trade with the Argentine Republic is carried on, the 

 certificates required by the Republic being supplied from 

 the station. Another important institution is the Station 

 for Plant Protection, founded some seven years ago as a 

 means of protection for the vineyards and orchards of 

 Germany against the San Jos^ scale insect and other pests 

 liable to be imported into Germany on American apples, 

 fruit-trees, &c. This station is in charge of Dr. Brick, 

 who, armed with the necessary staff, library, and apparatus, 

 must report on every barrel of apples coming into port. 

 The rejected apples, dangerous to Germany, find a ready 

 market in England and elsewhere. 



In the Botanical Museum the collections are arranged 

 in two sections. One part follows the usual lines — the 

 specimens are arranged in systematic order, according to 

 their natural affinities, and serve more especially for educa- 

 tional purposes. The other part of the collection appeals 

 to commercial interests. The fibres of commerce, the chief 

 rubbers, gums, resins, cereals, &c., are in each class 

 grouped together, regardless of natural affinities, and solely 

 for trade purposes. .\ new and more commodious museum 

 in the Botanic Gardens is just reaching completion. The 

 museum is regularly visited by schools and their teachers, 

 and a large piece of ground is set apart in the suburbs 

 to supply the specimens required in the schools for teach- 

 ing purposes. 



Everything that could be was done by the local 

 botanical staff and others to make the meetings of the 

 societies a success. The Hamburg Government granted a 

 sum of 4000 marks toward expenses, and in other different 

 ways showed a practical interest in the proceedings. One 

 important feature was the first International Conference on 

 Seed Testing. Most of the seed stations in the world were 

 represented, and attempts to establish a uniform system 

 of testing, applicable in different countries, were discussed. 

 It was generallv felt that it would be premature to seek to 

 go further at present than simple discussion. Many valu- 

 able papers were contributed. Dr. Stebler gave the results 

 of twenty years' investigation in the station at Zurich as to 

 the countrv of origin of the seeds of commerce, judged 

 sometimes from the particles of soil found in the impuri- 

 ties (!), but more usually from the weed-seeds present. 

 This paper was fully illustrated by dried plants and seeds. 

 Dr. von Weinzierl, of Vienna, dealt with sugar-beet and 

 mangel seeds: Dr. Degen, of Budapest, with dodder in 

 clover ; Prof. Rodewald. of Kiel, with the sources of error 

 in seed-testing ; while Prof. Voigt, of Hamburg had pre- 



NO. 1929, VOL. 74I 



