Y I 



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PERIODIC ARRANGEMENT OF ELEMENTS 173 



as well as from those, of thorium, are produced by gases 

 resembling in their inertness the gases of the argon group. 

 These gases, moreover, have the extraordinary property 

 that they are transient, although they change in very 

 different intervals of time. Whereas the gas from thorium 

 is half gone in about a minute (that is, has changed to 

 the extent of one-half into some other substance or sub- 

 stances), that from radium requires about four days before 

 it has undergone half the change of which it is capable. 

 A third gas has been obtained from a radio-active element 

 to which the name ' actinium ' has been given by its dis- 

 coverer, Debierne; this gas has an extraordinarily short 

 life, for the total duration of its existence is only a few 

 seconds. The spectrum of the gas from radium has been 

 mapped by Ramsay and Collie ; the amount of gas pro- 

 duced from a known weight of radium bromide has been 

 measured by Ramsay and Soddy; and they, too, proved 

 that one of its products of decomposition is the lightest 

 gas of the argon group, helium. At first, the spectrum of 

 the emanation from radium shows none of the character- 

 istic lines of helium ; but in the course of a few days the 

 helium spectrum appears in full brilliancy. Here, evi- 

 dently, is a case of the transformation of one element into 

 another ; no doubt there are other products than helium, 

 but what they are remains for the present unknown. If 

 they were elements like iron, for example, there are at 

 present no known means delicate enough to detect the 

 extremely minute amount which would be produced. 

 These gases from radium, thorium, and actinium are self- 

 luminous, and shine brilliantly in the dark ; and they also 

 possess the power of altering air and other gases with 

 which they are mixed, so that they acquire the property 

 of discharging an electrified body; the air is said to be 

 ' ionised.' But a still more remarkable property is their 

 giving off heat during their change into other elements, 



