282 Sir J. Dewar on Low Temperature Studies [Jan. 16,1920 



and is given in the following table (without any correction for re- 

 flection) : — 



Piatt 



Silver chloride 

 Silver bromide 

 Silver iodide... 



Thickness 



0*15 to 1 mrn. 



0*5 to 1 mm. 



1-2 ram. 



Transmitted proportion 



84% to 85% 



83% to 86% 



65% 



A certain number of observations were made with the black body 

 source cooled by boiling solid carbonic acid. The values obtained 

 were not very different from those already given. The following- 

 table gives comparative results for a few typical bodies : — 



Observations of the emissivity of various substances, as polished 

 plates, placed above the thermoscope gave the following values in 

 comparison with the black Leslie cube : — 



Emissivities at 100° C. 

 Thermoscope Observations. 



'•Black Body" 



Iron ... 



Lead ... 



Tin 



Aluminium 



Zinc ... 



Nickel Silver 



Pewter 



Copper ... 



Brass ... ... ... 



Mercury ... ... 



100 



14 



14 



12 



9 



7 



6 



5 



5 



5 



3 



Acknowledgment is due to Mr. W. J. Green, B.Sc, for the 

 valuable assistance he rendered during the progress of the work and 

 in the preparation of this abstract. 



[J. D.] 



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