776 SCIENCE PROGRESS 



that the atoms of the molecule of a compound gas are not charged with electricity 

 of opposite signs, but each atom contains as much positive as negative electricity, 

 a result of great importance for chemistry. 



The method is a very powerful one for finding the weights of the atoms and 

 molecules present in the tube, and has already led to the announcement of 



a new atom and a new molecule. A parabola for which — = 3 (taking — =1 for 



the singly charged hydrogen atom) is attributed to triatomic hydrogen. A 



parabola for which — = 22, which accompanies the neon parabolaf — = 20 J, indicates 



an atom of weight 22. Mr. F. W. Aston has partially separated such a gas 

 from neon by diffusion ; differences of density in the two components have been 

 actually measured. 



Besides the parabolic curves discussed already, there appear on the photo- 

 graphs at lowest pressure straight lines, which the author calls " secondaries." 

 Their origin is the subject of an interesting theoretical discussion, in the course 

 of which the conclusion is reached that the minimum velocity required by an 

 electron to ionise an atom of hydrogen is 1 1 volts ; this is the value obtained 

 by Lenard in 1903, one of whose students, F. Mayer, has recently redetermined 

 it to be u'5 volts. The discussion on p. 70 of the amount of ionisation produced 

 by cathode rays is not very clear, as no explicit mention is made of the fact 

 that there is a certain best velocity of the primary electron which produces the 

 most secondary electrons, and the matter is further obscured by the misprinting 

 of "increases" for "decreases" on p. 71. In this connection reference may be 

 made to a paper by C. Ramsauer in the Jahrbuch der Radioactivitat, ix. 1912, 

 p. 515. 



Enough has been said to indicate the extraordinary interest of the researches 

 described. The book, further, contains short accounts of the retrograde and 

 anode rays, of Stark's experiments on the Doppler effect in canal rays, and of 

 experiments on the continuous production of helium and neon by bombardment 

 by cathode rays, affirmed by Sir William Ramsay. Sir J. J. Thomson does not 

 pronounce definitely in favour of any particular source of the gases so liberated. 



The book unfortunately contains many oversights. The figures are not 

 always clear ; for instance, fig. 50 is not marked with the letters given in the 

 text, and fig. 29 likewise. There are also misprints, such as " Kulschewski ' : for 

 " Kutschewski," and misplaced commas sometimes produce odd effects, as in 

 the many cases in the index where Doppler (spelt Doppler throughout) is made 

 to appear as part of the name of the man who has worked on his effect. These 

 are trifles, however. The importance of the book is obvious, and a book by 

 Sir J. J. Thomson requires no recommendation. 



E. N. da C. A. 



Practical Exercises in Heat. By E. S. A. Robson. Second Edition. [Pp. 

 xii + 213.] (Macmillan & Co., 1913. Price 3^. 6d. net.) 



This book, written by one evidently experienced in teaching, contains accounts 

 of a number of varied experimental exercises in heat, which are described clearly 

 yet briefly. They are none of them very difficult, all being well within the ability 

 of first and second year men ; the range is, however, wide, and includes many 

 interesting experiments not usually described in books of this kind, such as 

 simple determinations of the calorific values of fuels, and the use of the resistance 



