On the Velocity of Electric Waves in Air. 1 15 



If, then, a substance have a maximum density at any par- 

 ticular temperature, <r vanishes for that temperature, and 

 we get 



But this is not all. The first law of thermodynamics, 

 written in its ordinary form, is 



at the maximum density point I ^— - ) +p vanishes, as we 

 have just seen ; consequently 



J.*J-(g) # .4T. 



If fZT be zero e?Q vanishes ; in other words, no heat is absorbed 

 or evolved in any isothermal transformation at this temperature ; 

 i. e. the latent heat of isothermal transformation is zero. 

 Similarly dT vanishes if dQ be zero. 



Many interesting conclusions may be drawn from this 

 remarkable result ; among them are the following : — 



1st. The Joule-Thomson effect is zero for every substance 

 at its maximum density, just as it is for an ideal perfect gas, 

 though for a very different reason. 



2nd. The infinite number of specific heats which every 

 substance possesses is reduced at this point to one. 



University of Melbourne, 

 December otk, 1898. 



VI. Velocity of Electric Waves in Air. 

 By G. V. MacLean *. 



[Plate I.] 



HERTZ determined the wave length in air, in one of his 

 experiments, to be 9'6 m., with an antenode 70 cm. 

 behind the reflector. In the case of electric waves along 

 wires, he found the rate of propagation to be 2*8 X 10 10 cms. 

 per sec. He further proved, if slow oscillations were used, 

 that the length of the electric waves along wires and in air 

 without wires would differ, but if rapid oscillations were 

 employed the lengths of the waves would not differ. The 

 truth of this has been confirmed by J. J. Thomson and 

 Lecher. 



* Communicated by Prof. Trowbridge. 



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