March 2, 1905] 



NATURE 



413 



Pannett, demonstrates that practical classes in 

 physiology are not confined to universities and 

 colleges of university standard. The book itself is 

 not a serious contribution to scientific literature, and 

 its authors have neither the requisite training nor 

 knowledge to make it such. It is a mere compilation 

 or rechauffi from other well known te.xt-books. One 

 notes that one of the authors blazons upon the title- 

 page that he has obtained a scholarship at the 

 inter. M.B. examination at the University of London, 

 and this is a fair index of what the reader may 

 expect in the interior of the volume. A note-book 

 carefully kept by any moderately good medical student 

 would be equally worthy of publication. 



W. D. H. 



Laboratory Notes on Practical Metallurgy : being a 

 Graduated Series of Exercises. Arranged by Walter 

 Macfarlane, F.I.C. Pp. x-(-i40. (London: Long- 

 mans, Green and Co., 1905.) Price 2S. 6d. 

 I'lHs little book is apparently intended as a first 

 course for beginners in practical work in a metal- 

 lurgical laboratory, and especially for those who are 

 preparing for the examination of the Board of 

 Education in stages i and 2 of practical metallurgy. 

 For these classes of students it will be useful and 

 deserves commendation. 



It consists of a series of practical exercises, all 

 well within the grasp of the average boy, graduated 

 ■ ind arranged with the view of developing the habit 

 of observation, and the instructions given for doing 

 ihem show a much more intimate acquaintance with 

 (he simpler operations of a metallurgical laboratory 

 than is generally found in works of this class. In 

 Ihe first eighteen pages the student is introduced to 

 furnace work by simple experiments on the melting 

 i>f metals under various conditions, to prepare him 

 for the subsequent more difficult operations. 



The preparation of the ordinary common alloys 

 follows, and then a series of well-chosen exercises 

 illustrates the oxidation of metals and the reduction 

 of metallic oxides and sulphides. Later, the more 

 complex subject of the principles on which the pro- 

 cesses for the extraction of copper, lead, gold, and 

 silver from their ores depend is dealt with. 



The book concludes with a few elementary exercises 

 in assaying gold and silver ores, and the analysis of 

 coal and coke. In the appendix are several tables, 

 ihe most important being one giving the percentage 

 composition of some of the common alloys. 



There are a few slips and blemishes in the text, 

 hut they are for the most part trivial, one of the 

 chief being in the table just mentioned, in which 

 Ihe composition of the British gold coinage is given 

 as gold 91.66, silver 8-33; the latter should of course 

 be "copper." The book contains much useful in- 

 formation for junior students, and can be recom- 

 mended for their use. 



Lc Liege. Scs produits el ses sous-produits. By M. 



Martignat. Pp. 158. (Paris : Gauthier-Villars and 



Masson et Cie.) Price 2.50 francs. 

 The latest addition to the " Encyclop^die Scientifique 

 des Aide-M^moire " is divided into two parts. The 

 first part is concerned with the formation of cork in 

 Ouercus suber, the distribution of the tree, its treat- 

 ment, its maladies and enemies, &c., and concludes 

 with an account of prices and other commercial con- 

 siderations. The second part describes how the natural 

 product is treated in the manufacture of corks of 

 all kinds, and how it is utilised in the production of 

 linoleum and other materials. 



NO. 1844, VOL. 71] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by liis correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for tliis or any other part of Nature. 

 No notice is talien of anonymous communications.] 



Charge carried by the a Rays from Radium. 



No special difliculty has been experienced in showing that 

 tiiR particles (electrons), expelled from radium, carry with 

 tliem a negative charge of electricity. The positive charge 

 left behind on the vessel containing the radio-active material 

 IS simply and strilcingly illustrated in the arrangement de- 

 vised by Strutt, which is now popularly known as the 

 " radium clock." 



Since the a particles are deflected by 3 magnet as if the) 

 carried a positive charge, it is to be expected that this 

 charge should be easily detected ; but all the initial experi- 

 ments made for this purpose resulted in failure. Since there 

 are four products in radium which give out a particles, and 

 only one which gives out particles, it is theoretically to 

 be expected that four ct particles should be expelled from 

 radium for each /3 particle. 



In the Bakcrian lecture {Pliil. Trans., series A, vol. cciv., 

 p. 212, 1904) 1 described some experiments that were made 

 10 determine the charge carried by the o particles. About 

 half a milligram of radium bromide was dissolved in water, 

 and spread uniformly over a metal plate and evaporated to 

 dryness. With a plate of 20 sq. cm. in area, the absorp- 

 tion of the a rays in the thin film of radium bromide is 

 negligible. The solution of the radium released the emana- 

 tion, and, several hours after removal, the activity of the 

 radium fell to about one-quarter of its maximum value, and 

 the /3 and y rays from it practically disappeared. The ex- 

 periments were made with the radium film at this mini- 

 mum activity, in order to avoid the complication which 

 would ensue if the $ particles were present. An insulated 

 plate was placed parallel to the radium plate and about 

 3mm. away from it. The apparatus was enclosed in an 

 air-tight vessel, which was exhausted to a very low vacuum. 

 The current between the plates v/as measured by an electro- 

 meter. The saturation current between the plates rapidly 

 fell with decrease of pressure, but soon reached a limiting 

 value — about i/rooo of the initial — which could not be re- 

 duced further, however good a vacuum was obtained. No 

 certain evidence that the o particles carried a positive charge 

 could be obtained. It was thought possible that the in- 

 ability to reduce the current below this value might be 

 due to a strong secondary radiation, consisting of slow- 

 moving electrons, which were liberated by the impact of the 

 a particles on matter. Strutt (Phil. Mag., August, 1904) 

 has also observed a very similar effect, using a plate of 

 r.idio-tellurium, which is well suited for this purpose, as it 

 gives out only a rays. 



J. J. Thomson (Proc. Camb. Phil. Soc, November 14, 

 1904; see Nature, December 15) has recently shown in a 

 striking manner that a large number of slow-moving elec- 

 trons are liberated from a plate of radio-tellurium, although 

 this substance is supposed to emit only o particles. These 

 electrons could be readily bent back to the plate from which 

 they came by the action of a magnetic field. No indication, 

 however, that the o particles carried a charge was 

 obtained. 



I have recently attacked this problem again, using the 

 methods and apparatus previously described, but, in addi- 

 tion, employing a strong magnetic field to remove the 

 slow-moving electrons present with the o particles. The 

 apparatus was placed between the pole-pieces of an electro- 

 magnet, so that the field was parallel to the plane of the 

 plates. In such a case, most of the escaping electrons de- 

 scribe curved paths and return to the plate from which they 

 set out. On application of the magnetic field, a very striking 

 alteration was observed in the magnitude of the current. 

 The positive and negative currents for a given voltage were 

 greatly reduced. The upper plate, into which the a par- 

 ticles were fired, rapidly gained a positive charge. In a 

 good vacuum, this was the case whether the lower plate 

 was charged positively, or negatively, or connected to earth. 

 The magnitude of the charge, deduced from these expert- 



