o32 Mr. A. H. Davis on Natural 



value of this variable appears to depend on the number of 

 atoms in the molecule, and has the* same value for the diatomic 

 gases oxygen, hydrogen, and nitrogen as it has for air, so 

 that these gases should give convection results in agreement 

 with the curve for air. 



(3) Method of examination of data. 



Published data have been examined in the light of the 

 formula, and to obtain a convenient tabular summary the 

 following general procedure has been adopted. 



The published data have been converted into c.g.s. units, 

 corrected for radiation if necessary, and plotted on a graph 

 from which values have been read off for even values of 

 temperature excess. This gives corresponding values of 

 Qd? and H. 



To obtain 0d 3 /v 2 and H/& it was necessary to decide on 

 the values of k and v appropriate to the particular experiment. 

 For a hot wire in air at room temperature (15° C.) the 

 appropriate values are neither those for air at the temperature 

 of the hot wire nor those for air at 15° 0. As a first 

 approximation the values actually taken were those for air 

 at the mean of these two temperatures. 



In cases where the pressure was not atmospheric allowance 

 has been made as before, and in addition it has been assumed 

 that, as indicated by the kinetic theory of gases, both the 

 conductivity k and the viscosity rj are independent of 

 pressure, and consequently- the kinematical viscosity v has 

 been taken as inversely proportional to the pressure of the 



s as - 



In following this general procedure certain auxiliary 

 constants have been used, as explained in the following 

 paragraphs : — 



(a) Radiation correction. — Data for heat losses are often 

 published in the form of a total loss, radiation being included. 

 Langmuir *, who used platinum wires, published his data 

 with the radiation eliminated, and in this paper results 

 obtained by other experimenters for platinum surfaces have 

 been corrected, using his values for the emissivity for 

 radiation. For steam pipes, total heat losses given by 

 Petavel have been corrected by the amount attributed by 

 him to radiation, an amount equivalent to 0'4-XlO -12 

 x (T 4 — T 4 )| calories per sq. cm. per sec, where T and T 



* Langmuir, Pbys. Rev. xxxiv. p. 401 (1912). 



t 5xlO- 10 (T 4 — To 4 ) B.T.U. per square foot per hour. 



