June 13, 1901] 



NATURE 



the publication of the fact that Mr. Kelsall and Mr. Dale, 

 secretary of the Hants Field Club, might be able " to accompany 

 the party on one or more of its rambles " a sufficient guarantee 

 that the rights of wild plants would be respected. 



Furthermore, on the title page of the vacation course pro- 

 gramme, p. 9, and printed in conspicuous black type, is the 

 following notice : " Members of the party will, of course, 

 refrain from uprooting rare or scarce specimens." Yet Prof 

 Miall alleges " there was no such restriction in the printed 

 programme " ! 



In the daily itinerary as printed in the programme reference 

 is made to the character of the scenery, the soil and surface 

 geology, the prevailing vegetation, and to some of the rare 

 plants growing in the neighbourhood. From what we have 

 already shown it could hardly be our intention to raid these rare 

 plants, and especially as several of those mentioned will be out 

 of flower in August. Indeed, so particular are we in these 

 rambles that the needless uprooting even of the commonest 

 weed is discountenanced, as may be seen in the further notice 

 on p. 3 of the programme. 



In comparison with such a particularly odious charge as plant 

 exterminalion, the other strictures of your correspondent's letter 

 are, of course, scarcely worth noticing ; yet even with respect 

 to these I cannot resist pointing out that Prof. Miall's state- 

 ments are strangely at variance with the actual facts. For 

 example, he writes : " It is enough to condemn the programme 

 as an educational project that novices knowing little or nothing 

 of field -botany are set to study the subspecies of brambles." 

 But does the programme so recommend ? It distinctly says in 

 reference to this (p. iS), that " their identification will give 

 capital exercise in critical observation to the more advanced 

 worker. ^^ 



The real object of these field-studies, as stated on the front 

 page of our programme, is to give teachers "an insight into 

 the way in which plants grow, especially in their relations with 

 their environment — the influence of external conditions, such 

 as light, heat and moisture, upon their form, the mutual rela- 

 tionships between plants and animals and the influence of one 

 organism upon another," and is in no way connected with 

 collecting in the sense used by Prof. Miall. The vacation 

 students have varied interests — flowering plants, alga;, leaf- 

 fungi, &c., and the evenings are to be spent in discussing " the 

 most interesting of the objects collected " and on the " preser- 

 vation " of such as may be useful for class-work in the winter 

 courses. Readers of Nature will understand that work of 

 this sort does not mean the collection of rare flowering plants. 



Perhaps because of the peculiar gravity of the charge I may, 

 in conclusion, be allowed to introduce one personal note into 

 the reply. I should like to say that although I have conducted 

 field studies in botany for the last twelve years (including two 

 summer courses at the New Forest), yet, as it happens, I am no 

 collector myself, and have never made what botanists would 

 call a collection of dried plants in my life. Furthermore, I have 

 never possessed, or even "coveted," a single specimen of a rare 

 British plant. On the contrary, my sympathies are, of course, 

 entirely with those who are opposed to any interference with 

 our native flora, and I do most strongly protest against this 

 attempt of Prof. Miall to connect in any way whatever our 

 botanical work with such objectionable practices. 



I should be glad to send a copy of the programme to any one 

 who may care to see it. David Housto.v. 



County Technical Laboratories, Chelmsford, June lo. 



Emanations from Radio-active Substances. 



In a recent number of the Comptes rendus of the Paris 

 Academy (March 25) an account appeared by MM. P. Curie 

 and .\. Debierne of the production of a radio-active gas from 

 radium. In their experiments some radium was placed in a 

 glass vessel and the air exhausted by means of a mercury pump. 

 It was found that the vacuum steadily decreased, due to the 

 giving off of a gaseous substance from the radium. A small 

 amount of the gas thus collected was found to be strongly 

 radio-active. It caused phosphorescence in the glass tubes over 

 which it passed, and in course of time blackened them. Sub- 

 stances exposed in the gas became themselves temporarily 

 radio-active. 



Some time ago (Phil. Mag., January and February 1900) I 

 showed that thorium compounds continuously emitted radio- 



NO. 1650, VOL. 64] 



active particles of some kind, which preserved their radio-activity 

 for several minutes. This emanation possessed the remarkable 

 property of causing all bodies, in contact with it, to become 

 themselves radio-active. In an electric field the excited radio- 

 activity could be concentrated and confined to the negative 

 electrode. In this way I was able to make a fine platinum 

 wire become a very powerful source of radiation. 



The excited radio-activity gradually diminished, falling to half 

 its value in about twelve hours. The specimen of impure 

 radium then in my possession gave out no emanation and caused 

 no excited radio-activity. Later, Dorn, using the same methods, 

 showed that a preparation of radium from P. de Haen, 

 Hanover, gave out an emanation similar in properties to 

 thorium. With a specimen of radium obtained from the same 

 source I have found that the emanation given oft' is small at 

 atmospheric temperature, but can be enormously increased by 

 slightly heating the radium. In this way I have obtained ten 

 thousand times the amount of emanation given off at ordinary 

 temperatures. An account of these experiments is given in the 

 Physikalisihe Zeilschrtft (April 20). 



By passing the emanation with a current of air into a closed 

 vessel, and then closing the openings, the emanation remains radio- 

 active for a long time. The radioactivity decreases slowly, but 

 is still quite appreciable after an interval of one month, M. 

 and Mme. Curies, some time ago, stated that they had obtained 

 a radio-active gas which preserved its activity for several weeks ; 

 this is possibly identical with the emanation. 



Up to this point I had been unable to obtain any definite 

 evidence whether the so-called emanations were vapours of the 

 radio-active substances, radio-active gases, or radiating particles 

 large compared with a molecule. The radium and thorium, 

 when placed in an exhausted tube, gave no appreciable lowering 

 of the vacuum, and no new spectral lines could be observed. 

 The quantity of substance emitted was too small to examine by 

 chemical methods. 



Quite recently, however, some light has been thrown on the 

 question of the nature of these emanations by examining their 

 rate of difi'usion by an electrical method. In these experiments 

 I have been assisted by Miss H. T. Brooks, and the results 

 point to the conclusion that the emanation from radium is in 

 reality a radio-active gas, with a molecular weight probably 

 lying between 40 and 100. 



There is one distinct feature which distinguishes the emana- 

 tions from radium and thorium. The thorium emanation loses 

 its radio-activity in a few minutes, while the excited radio- 

 activity due to it lasts several days. The radium emanation, on 

 the other hand, preserved its radiating power for several weeks, 

 while the excited radioactivity due to it disappears in a few 

 hours. In the following experiments it was only possible to 

 experiment with radium emanation, on account of the rapid 

 decay of radio-activity of the thorium emanation. 



The diffusion apparatus was similar to that which had been 

 employed by Loschmidt in 1870 in his determinations of the 

 coefficients of interdififusion of gases. 



A brass cylinder, 73 cm. long, 6 cm. in diameter, was divided 

 into two equal parts by a metal slide, which could be opened 

 or closed. The ends were closed by insulating ebonite stoppers, 

 through which passed central rods half the length of the tube. 

 In order to introduce the emanation into one half of the cylin- 

 der the slide was closed, and a slow current of air, which had 

 passed over slightly heated radium and thus carried the emana- 

 tion with it, was passed through the cylinder. When a sufficient 

 amount had been introduced the current of air was stopped and 

 the openings closed. After standing for an hour or more the 

 slide was opened, and the radio-active emanation slowly dif- 

 fused into the other half of the cylinder. The amount of emana- 

 tion in each half of the cylinder after any interval was tested by 

 observing the current through the gas, when a suitable P. D. 

 was applied, by means of an electrometer. The current is 

 carried by the gaseous ions which are continually produced by 

 the radiation from the emanation. From these observations 

 the coefficient of inter-difi'usion of the emanation into air at 

 atmospheric pressure and temperature can be readily deduced. 

 The experiments are, however, complicated by the excited 

 radio-activity on the electrodes, which must be taken into 

 consideration. 



So far as the observations have gone up to the present, the 

 coefficient of diftusion of the emanation into air has a value be- 

 tween o'lo and 0'I5, and probably nearer the former. Now the 

 coefficients of inter-dift'usion of some known gases and vapours 



