REVIEWS 777 



thermometer. The experiments on the thermocouple are very simple and neat. 

 We must, however, take strong exception to an experiment described as measuring 

 the temperature of the blowpipe flame ; a brass cylinder is heated in the flame, 

 and its final temperature, measured calorimetrically, taken as being the flame 

 temperature. This is, of course, wildly wrong ; the melting point of brass is 

 about 900° C, while the true temperature of the ordinary Bunsen flame goes from 

 1400° C. up to i8oo°C. The student could easily convince himself that the brass 

 is only prevented from melting by radiation losses, and other disturbing factors, 

 by fusing thin brass and iron wires in the flame ; this would be more instructive 

 as to flame temperatures than the experiment described. With this exception we 

 have only found trifling faults in the book, which is on the whole to be recom- 

 mended. The working out of actual numerical cases is helpful to the student, 

 and there are some useful tables at the end of the book. 



E. N. da C. A. 



Photoelectricity. By H. Stanley Allen. [Pp. vii + 221.] (Longmans, 

 Green & Co., 191 3. Price js. bd. net.) 



In 1887 Hertz observed that the passage 01 a spark was facilitated it ultraviolet 

 light fell on the spark gap ; and in the next year Hallwachs found that such light 

 possessed the power of discharging plates of certain metals if they were negatively 

 charged, but not if they were positively charged. In 1899 Lenard, and a few 

 months later J. J. Thomson, showed that the action of the light was to set free 

 electrons from the metal thus illuminated ; this effect of light in liberating 

 negative electricity has received the name of the photoelectric effect. (It may 

 be noted here that Lenard's fundamental paper first appeared in the Sitztings- 

 berichte der Kaiserlichen Akademie in Wien, v. 19, October 1899 ; it was reprinted 

 in the Annalen der Physik in the following year. The paper is quoted by the 

 latter date only in the book under review and other English books. The point 

 is of some importance as regards priority.) 



Dr. Allen undertakes to give an account of the work, very extensive in 

 recent years, which has been carried out on the subject ; and he has added 

 chapters on the connected subjects of Phosphorescence and Photochemical 

 Action. The chapter on phosphorescence is very welcome, as the recent work 

 in this field has been much neglected in English text-books, and is very important 

 for the information it affords on the mechanism of light emission. As regards 

 the photoelectric effect itself, it is remarkable, considering the number of papers 

 published, how little definite information has been won beyond that contained 

 in the early papers of Hallwachs, Elster and Geitel, and, especially, Lenard. So 

 much contradictory and indefinite work has been done of recent years that the 

 task of arranging it in a clear and connected form is one of great difficulty ; if 

 Dr. Allen has not always succeeded in ordering the material and criticising 

 it so as to make clear what are the most reliable results at the present time, 

 he has, in general, given good summaries of the results of the individual 

 experimenters. At the same time we do not think that the amount of space 

 devoted to the different researches is always well chosen ; the work of Hughes, 

 which is not very conclusive {e.g. the distilled metal surfaces do not seem to give 

 such very satisfactory results as Dr. Allen frequently states. See, for criticism 

 on this and other points, a paper by Pohl and Pringsheim, Phil. Mag. December 

 1913), is treated at very great length, while Lenard and Ramsauer's extensive 

 work on the photoelectric effect in gases, which is not very accessible to English 



