56 



KNOWLEDGE. 



February, *913. 



microscope of magnifying power in the neighbour- 

 hood of fifty. A good combination is a Leitz No. 

 eye-piece with a No. 4 or No. 3 objective, while it is 

 better to have the screen very slightly illuminated so 

 that the eye may be continuously focused on it. 

 The source is arranged so that the scintillations 

 appear at a rate not greater than about ninety per 

 minute and not less than about five per minute, these 

 rates being generally the limits for accurate counting. 

 In some cases it is necessary to have the source and 

 screen in a vessel at reduced pressure owing to the 

 limited "range" of the a particles. The scintilla- 

 tions are timed with a stop watch. They do not 

 appear regularly but are distributed according to 

 chance, and to obtain an accurate estimate a large 

 number of scintillations must be counted — in fact, 

 the scintillation method has given an interesting 

 experimental confirmation of the laws of probability 

 as applied to radioactive disintegration. 



Perhaps one of the most interesting experiments 

 with scintillations is that originally used by Geiger 

 and Marsden in the detection of the short-lived 

 a ray products after thorium and actinium 

 emanations. Thorium emanation is a gaseous 

 radioactive product emitting a particles and 

 followed by a second a ray product whose mean life 

 is only one-fifth of a second, while actinium 

 emanation is similarly followed by an u ray product 

 of mean life only one three hundred and fiftieth of a 

 second. Figure 64 shows a " drawing room " modi- 

 fication of the apparatus designed by Mr. F. H. 

 Glew. 



Sj and S 2 are two zinc sulphite screens lying one 

 above the other and separated by thin paper strips K.K. 

 The upper screen is very thinly coated so that it is 

 practically transparent and scintillations on the lower 

 screen can be seen from above. M is a source of thorium 

 emanation consisting of a very small amount of meso- 

 thorium. The emanation diffuses beween the screens 



in an amount regulated by the small strip P. 



Consider an atom of the emanation which disinteg- 

 rates between the screens emitting an a particle which 

 produces a scintillation on either the upper or lower 

 screen. The atom of thorium A produced from this 

 emanation atom has an expectation of life of one-fifth 

 of a second before in its turn it disintegrates, and gives 

 off a second a particle which causes a scintillation on 

 either Sj or S 2 . Thus the result is two scintillations, 

 one following the other with about one-fifth of a 

 second interval. These scintillations appear on about 

 the same area of the screens and can be observed by 

 an ordinary pocket magnifying glass or microscope. 

 In the case of actinium emanation the scintillations 

 are given off with an average interval of one three 

 hundred and fiftieth of a second, so that the interval 

 cannot be distinguished by the eye, the result being 

 apparently two scintillations at the same time, 

 generally very slightly separated in position. 



The scintillation method has also had an impor- 

 tant application in the study of the scattering of 

 a particles by matter. When a parallel pencil of 

 a particles is incident on a thin metal foil, the indi- 

 vidual a particles suffer deflection by the atoms with 

 which they come in contact and the beam as a con- 

 sequence becomes scattered. The distribution of 

 a particles in the scattered beam can be observed by 

 scintillations and from this distribution considerable 

 evidence can be drawn as to the electrical forces 

 inside the atoms causing the scattering. Thus it 

 has been shown that all atoms are constituted in some 

 respects similar to small models of our solar system. 

 They appear to have a very strong central charge 

 consisting of a number of elementary charges equal 

 to half the atomic weight. This central charge 

 appears to be concentrated within a volume extremely 

 small compared with the size of the atom and to 

 be surrounded by electricity of the opposite sign 

 which on our analogy may be compared with planets. 



A PROPOSED BRITISH ECOLOGICAL SOCIETY. 



The British Vegetation Committee, which was founded in 

 1904, has up to the present consisted only of active workers 

 in Plant Ecology, and has met with a very large measure of 

 success. In fact, it has had to be enlarged, and includes so 

 many " associate members " that a new Society is being formed 

 to take its place and push forward its work. It is felt that the 

 organisation of a Society with a regularly published and care- 

 fully edited journal might succeed also in bringing the most 

 scattered workers — many of whom are still outside the ranks 

 of the Committee — into touch with one another, and would 

 keep them informed of the progress of the subject. Such a 

 Society might also perform a similar function for the many who 

 are keenly interested in Ecology without themselves being 

 active workers in the field. 



It is suggested that it should be called the British Ecological 

 Society, and that the annual subscription should be one 

 guinea, and it must be pointed out that it is intended to have 

 a paid Secretary and Editor, and to publish a quarterly journal 

 containing to begin with twelve thousand words in each 

 number, which would be sent post free to members, and sold 

 to non-members at 3s. per number. 



From a circular which has been issued by the British 

 Vegetation Committee, and signed by Dr. W. G. Smith, of 

 the Glasgow and East of Scotland Agricultural College, and 



Mr. A. G. Tansley, M.A., University Lecturer in Botany 

 Cambridge, we further learn that the Society would meet 

 once or twice a year in different centres, for two or three 

 days, and that the journal would include articles and notes on 

 methods, on special points of importance, reports of pro- 

 ceedings of the Society, with accounts of Ecological work in 

 progress. It would also make a feature of prompt reviews of 

 all important recent publications, on British vegetation as well 

 as foreign work which has a bearing on British Ecology. 



The price of the journal and the rate of subscription to the 

 Society mentioned above are to be regarded as provisional ; 

 if sufficient promises of support are forthcoming, it will be 

 possible to fix the subscription at much less than a guinea. 



It will be of the greatest assistance to the British Vegetation 

 Committee if readers of "Knowledge" who are interested 

 in this proposal will communicate, as soon as possible, with 

 the Secretary of the Committee, Dr. W. G. Smith, 9, Braidburn 

 Crescent Edinburgh, stating whether they are prepared to 

 join the proposed Society at a subscription of one guinea per 

 annum ; if not, whether they are willing to join at a lower rate 

 of subscription, stating the amount they would be willing to 

 pay ; and whether they have any criticisms or additional 

 suggestions to make with regard to the scheme proposed. 



