Radium Emanation at Low Temperatures. 729 



Table II. 



Deflexion before condensation 25*4 cm . 



Deflexion 4 hours after applying liquid air 0*8 „ 



On allowing the temperature to rise : — 



Temperature. 



Deflexion. 



— 177 



0-8 cm. 



-172 



0-8 



-164-5 



08 



-160-5 



1-15 



-154-5 



2-55 



-142-5 



7 65 



-131-5 



11-5 



-118-5 



13'6 



-110-5 



14-6 



Followed by along period o 



* very slow rise of ionization. 



The form of the curve can be very simply explained on a 

 basis of a normal behaviour of the emanation. 



Consider, first, curve A which corresponds to 78 m.r.e,, 

 and in which a galvanometer was used to measure the 

 ionization. The galvanometer being a very insensitive in- 

 strument required an enormous ionization to affect it. 

 Apparently there was no appreciable rise of ionization, and 

 therefore no appreciable volatilization of emanation until the 

 temperature approached — 163° G. To this is due the initial, 

 flat, portion of the curve. About —163° the emanation 

 began to volatilize in larger quantity, the vaporized emana- 

 tion distributed itself throughout the tube, and RaA and 

 Ra began to grow. As the temperature increased all three 

 products — the emanation, RaA, and RaC — increased more 

 rapidly in the upper part of the tube. The continuously 

 increasing number of a-rays sent out by these products 

 caused a corresponding increase of ionization in the chamber 

 above, thus giving the steep, rising, portion of the curve. 

 After the emanation has all volatilized the rate of increase of 

 emanation vapour in the top part of the tube must fall off 

 very greatly. On account of the low temperature at the 

 bottom of the tube, the density of the emanation is greater at 

 this part than in the upper part, although all the emanation 

 has volatilized. But as the bottom temperature continues to 

 rise slowly the density here decreases, and this causes a slow 

 transference of emanation molecules from the lower to the 



Phil. Man. S. 6. Vol. 21. No. 126. June 1911. 



B 



