Radium Emanation at Low Temperatures. 731 



the ionization measurements for quantitative calculations. 

 The slight ionizations at the beginning of each of the curves 

 shown were due to the 7 and some ft rays from the radio- 

 active products of the condensed emanation. 



The experiments show that a vapour phase corresponding 

 to condensed radium emanation can easily be traced to a 

 temperature as low as — 180° C. 



Gray and Ramsay*, as the result of an experiment in 

 which the opacity of the condensed emanation was the test 

 of solidity, state that the emanation solidifies at —71° C, 

 the vapour pressure then being 500 mm. Hg. Under the 

 infinitesimal partial pressures and low temperatures in the 

 present experiments the state of the condensed emanation is 

 not known, but whether it exists as solid, or as liquid, or as 

 an adsorbed layer, we should expect on a basis of behaviour 

 like ordinary gases under familiar conditions : 



(1) that at any temperature a vapour phase of the emana- 



tion would exist ; 



(2) that volatilization from the condensed to the vapour 



phase would set in as soon as the temperature com- 

 menced to rise ; and 



(3) that volatilization would proceed gradually, becoming 



more and more rapid as the temperature increased. 



The experiments described bear out these expectations, 

 and thus far the behaviour of the emanation may be said to 

 be normal. It can be seen that the temperature of final 

 volatilization from the condensing surface will depend on 

 the quantity of emanation, and therefore it cannot be said, 

 unless particular conditions are stated, that the emanation 

 when condensed will volatilize at any particular temperature. 



The experiments cannot tell us whether the phenomenon 

 of volatilization under these conditions is affected by surface 

 adsorption or adhesion. Such matters will be settled when 

 it can be shown definitely that in equilibrium with these 

 infinitesimal volumes of condensed emanation there is, or is 

 not, at any fixed, low, temperature an invariable value of 

 vapour pressure. Already we have from Russ and Makowerf 

 a few incidental observations which suggest that at the liquid 

 air temperature the amount of emanation vapour in equi- 

 librium with condensed emanation is not a fixed quantity 

 but depends on the amount of emanation condensed. 



Following the work already described in this paper, the 



* Loe. cit. 



f Le Radium, vi. L909, p. 182; Proo. Roy. ^00. A. lxxxii. p. 205. 



3 B 2 



