22 ROYAL SOCIETY OF CANADA 
emanating power of thorium increased with rise of temperature to about 
a red heat, but on heating to a white heat the emanating power was 
destroyed and could not be recovered. An examination of a specimen of 
radium obtained from De Haen, Hanover, showed that the effect of 
temperature on its emanating power was very large. When the substance 
was heated below a red heat, its emanating power increased over 10,000 
times, but was to a large extent destroyed by heating to a higher 
temperature. The emanation, obtained by passing a slow current of air 
over heated radium, was found to preserve its radioactive powers for 
weeks, when kept in a closed metal vessel. The radioactivity slowly 
decayed, but was still quite appreciable after a month’s interval. 
The question now arose, if any physical experiments could be devised 
to settle the problem as to whether the emanation was in reality a radio- 
active gas, driven off from the substance, or a vapour of the substance, or 
a material emission of particles much larger than molecules. Experi- 
ments on thorium showed that no appreciable volume of gas could be 
collected by leaving thorium oxide in a vacuum tube connected with a 
mercury pump. No new lines were observed in the spectrum of the gas. 
The amount of the emanation thus given off was thus too small to detect 
by its volume in this way, but the electrical conductivity produced by 
the emanation in the gas, with which it is mixed, is often very large and 
can be used as a measure of the amount of emanation present. The 
emanation gives out a type of radiation which ionizes the surrounding 
gas, When a strong electric field is applied, the current through the gas 
reaches a maximum value, and is then a measure of the total number of 
ions produced per second, multiplied by the charge on the ion. 
By determining the rate of diffusion of the emanation into air or 
other gases, using the electrical method, it is possible to obtain an 
approximate estimate of its molecular weight. The co-efficient of inter- 
diffusion of most of the simple gases have long been known, and the 
results show the coefficient of diffusion of one gas into another is inversely 
proportional to the square root of the product of the molecular weights. 
If therefore the coefficients of diffusion of the emanation into airis found 
to have a value lying between that of two gases A and B, we can conclude 
that the molecular weight of the emanation lies between the molecular 
weights of A and B. 
The apparatus employed was similar to that used by Loschmidt’ in 
his experiments on the coefficient of interdHfusion of gases in the year 
1871. 
Fig. (1) shows the general arrangement. A long brass cylinder AB 
6 cms. in diameter, 73 cms. long, was divided into two equal parts by a 
movable metal slide S. ‘lhe ends of the cylinder were closed with 

1 Wiener Akad., 1871. 
