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XLV. Notices respecting JSeiv Books. 



Collected Scientific Papers. By Professor John Henry Poynting, 

 F.R.S. Pp. xxxii + 768. Cambridge University Press. 1920. 

 37s. 6cl. net. 



T)ROBABLY few are aware of the wide range of studies 

 •*- covered by Poynting's researches. His fame was made in 

 1884 by papers on the transfer of energy in the Electromagnetic 

 Field (Phil. Trans. Roy. Soc. A) and on electric currents and the 

 electric and magnetic induction in the surrounding medium (Phil. 

 Trans. 188S). The Poynting vector specifies the direction and 

 magnitude of flow of the energy in an electromagnetic field ; it is 

 at right angles to both the electric and magnetic forces, and is 

 proportional to the product of these forces and the sine of the 

 angle between them. This vector has taken an important position 

 in any modern theory ; and it may be said also from the practical 

 point of view that no clear idea of the propagation of energy in 

 wireless telegraphy could have been obtained without it. 



But in three other directions at least Poynting made investi- 

 gations of fundamental importance. In 1887 his attention was 

 concentrated on the phenomena of change of state, and at other 

 times he came back to this and the allied problems of osmotic 

 pressure. He showed thermodynamically that vapour pressure 

 must increase with isothermal increase of pressure ; that the 

 lowering of freezing-point with pressure must depend upon 

 whether both phases are subject to the pressure or one phase 

 alone. Experiments on this latter point did not bear out very 

 well his. theoretic conclusions, but there is no doubt that he was on 

 the right lines ; the difficulty was in trying to reproduce the 

 theoretical conditions. With regard to the former point it is now 

 known to be one example of a very general theory of pressure- 

 influence in connexion with which much theoretical and experi- 

 mental work has been done. 



Much more importance attaches to his investigations concerning 

 the pressure of radiation. Along with Dr. Guy Barlow he estab- 

 lished the existence of a tangential force when light is incident 

 upon an absorbing surface, and also the existence of a torque 

 when light passes through a prism. In 1910 he showed that a 

 radiating body recoils from the radiation it emits. 



His fourth main subject was that of gravitation. First (1878- 

 1891) he undertook a difficult investigation into the mean density 

 of the earth using a balance method. Although his method was 

 in the end overshadowed by the exceedingly neat method adopted 

 by C. Y. Boys, yet his extreme skill and perseverance overcame 

 many of the difficulties which were minimised in the later 

 method. He also sought whether the attraction between two 

 quartz crystals depends upon their orientation ; no difference was 



