RADIUM AND ITS PRODUCTS 187 



known law that gases decrease in volume proportionally 

 to increase of pressure ; if the pressure is doubled, the 

 volume of the gas is halved, and so on. Now this was found 

 to be the case with the emanation ; hence the conclusion 

 that it is a gas, in the ordinary meaning of the word. 

 But it is a very unusual gas ; for not only is it luminous 

 in the dark, but it slowly contracts, day by day, until it 

 practically all disappears. It does not lose its luminosity, 

 however; what remains, day by day, is as luminous as 

 ever ; but its volume decreased, until after about twenty- 

 five days the gas had contracted to a mere luminous 

 point. What had become of the helium ? That was dis- 

 covered on heating the tube. It is well known that glass, 

 exposed to the radium emanation, turns purple, if it is 

 soda glass; brown, if it is potash glass. This is due to 

 the penetration of the glass by the electrons, which are 

 exceedingly minute particles, moving with enormous 

 velocity. When the emanation changes into helium, 

 the molecules of that gas are also shot off with enormous 

 velocity, although they move much more slowly than the 

 electrons. It is sufficient, however, to cause them to 

 penetrate the glass ; but on heating they are evolved, and 

 collect in the tube, and the volume of the helium can be 

 measured. It turned out to be three and a half times 

 that of the emanation. But as the emanation is probably 

 fifty times as heavy as hydrogen, all the emanation is 

 not accounted for by the volume of helium found; it is 

 almost certain that solid products are formed, which are 

 deposited on the glass, and which are radioactive. Up to 

 the present these products have not been investigated 

 chemically. 



It was possible, knowing the volume of the emanation, 

 and knowing also the volume which the radium would 

 have occupied had it, too, been gaseous (for a simple rule 

 enables chemists to know the volume which a given 



