﻿4(32 Dr. Norman Campbell on 



for carbon dioxide for various saturation pressures, together 

 with the accepted value for the gas at ordinary pressures. 

 It will be seen that the curve for the liquid is concave 

 upwards. The values shown for the liquid and gas are con- 

 nected together conjecturally by means of a dotted curve, on 

 the assumption that the mobility varies continuously through 

 the critical point. It is of some interest that the curve so 

 obtained is very nearly indeed a parabola with its axis 

 horizontal. 



3. I am not aware that it has ever been pointed out that 

 those liquids which have a small viscosity at ordinary tempe- 

 ratures have, in general, a great vapour-pressure. This law 

 holds perfectly for homologous liquids, and it holds, so far as 

 order of magnitude is concerned, in all other cases. It is 

 needless to multiply instances : a glance at Landolt and 

 Bornstein\s Tables exhibits the law at once; but, as- extreme 

 cases, may be mentioned that of glass (which is a super- 

 cooled liquid) with its very large viscosity and negligible 

 vapour-pressure, and that of liquid C0 2 , which has larger 

 vapour-pressure and a smaller viscosity ihan that of any other 

 liquid on which measurements have been made. 



XLIV. Ionization by Alpha Rays. 

 By Norman Campbell, Sc.Z>.* 

 1. TN two previous papers on "Delta Rays " f it was 

 JL shown that it was extremely improbable that the 

 delta rays had an initial velocity as great as that corre- 

 sponding to the 20 volts which had previously been accepted. 

 It was further shown that the speed of the rays, if it was 

 greater than a small fraction of a volt, must be independent 

 of the speed of the exciting rays, and nearly, if not 

 quite, independent of the material from which they are 

 emitted. These conclusions appeared of sufficient interest 

 and importance in their bearing upon the general problem of 

 the mechanism of radiation to make it worth while to attempt 

 to establish them on ;a firmer basis by experiments of a dif- 

 ferent nature. The following pages contain an account of 

 these experiments, which confirm all the conclusions put 

 forward already, and produce, as a side issue, some results 

 of a different nature. After the experimental evidence has 

 been explained, the bearing of it on the main problem under 

 discussion is considered. 



* Communicated by the Author. 



t Campbell, Phil. Mag. Aug. 1911, p. 276, and Jan. 1912, p. 46. 



