20 Determination of Chemical Constants. 



Thus, the value deduced from the experiments is nearer 

 the older value (as given by Planck, Vorlesungen ilber die 

 Tkeorie tier Warmestrahlung) 5*3 . 10~ 5 , rather than the 

 newer values which have been obtained more recently, 

 e. g. Gerlach {Ann. d. Phjs. 50. 3, p. 259, 1916) 5'9 . 10"' 12 

 watt, cm. -2 . deg -4 . 



Summary. 



Methods of calculating the chemical constant and the 

 conditions necessary for obtaining accurate results are 

 discussed. It is shown that the older formula by which 

 most of the constants were calculated leads to a result 

 without physical significance. The formula which should 

 be used requires a knowledge of the specific heat of the 

 substance (i. e. of the value (3v). 



The chemical constant of mercury is calculated and the 

 accuracy of the result is discussed. The constant is found 

 to be 1-820 ±0-032. 



This value leads to a value for C , the universal constant 

 of the theoretical relation — — - 1*5 log M, within the 

 experimental error. 



The chemical constants G of cadmium and zinc are 

 evaluated from the vapour-pressure measurements (Phil. 

 Mag. vol. xxxiii. p. 193, 1917), and the values calculated 

 using the theoretical value of C are found to lie within 

 the experimental error. 



The value of C for monatomic substances, the mean of 

 all values so far determined, is found to be —1*622. 



The latent heats of vaporization of zinc, cadmium, and 

 mercury are given for various temperatures, and . the 

 maximum value is found for all monatomic substances at 



a temperature - 7 v r = l'9b. 



There appears to be need for further experiment before 

 the data obtained by Langmuir and Mackay for the vapour 

 pressures of tungsten, molybdenum, and platinum at high 

 temperatures can be used for accurate determinations of 

 their chemical constants. 



Stephan's constant for full radiation a has been deduced 

 from the mean value of the constant C and is given as 

 5*27 . 10~ 5 erg . cm. ~ 2 . deg" 4 . This value appears to be 

 definitely lower than the more recently determined values, 

 within the experimental error, and is dependent only on 

 the vapour pressure, specific heat of mercury, gas constant, 

 and velocity of light. 



Clarendon Laboratory, Oxford. 

 Sept. 1st, 1919, 



