WHAT IS AN ELEMENT? 159 



atom. It is almost certain that radium salts continually 

 emit such rapidly moving particles, and it is known that 

 while doing so the temperature of the radium salt is some 

 degrees higher than that of the surrounding atmosphere ; 

 radium, therefore, is continually giving off heat. We are 

 wholly unacquainted with any similar change; these 

 properties are new. But we do know of compound sub- 

 stances which decompose with slight provocation, give off 

 a great amount of heat in doing so, and at the same time 

 are wholly converted into a large quantity of gases ; per- 

 haps the most familiar example is gun-cotton, of which 

 most of the high explosives used for blasting and in the 

 manufacture of modern gunpowder are made. The dif- 

 ferences between the two phenomena, moreover, are 

 sufficiently pronounced: gun-cotton decomposes almost 

 instantaneously, with explosive violence; radium salts 

 slowly ; gun-cotton requires to be started by the explosion 

 of a percussion-cap ; radium salts decompose spontaneously, 

 and the rate of decomposition, so far as is known, appears 

 to be independent of temperature; the amount of heat 

 evolved when gun-cotton explodes, though great in itself, 

 is' small in comparison with that evolved during the de- 

 composition of an equal weight of radium salt ; and it is 

 not known that any electrical phenomena accompany the 

 decomposition of gun-cotton. Still, it appears reasonable 

 to suspect that the two kinds of change may, after all, be 

 similar, and that the heavy atom of radium is decompos- 

 ing into the lighter helium atom. It is pretty certain 

 that helium is not the only substance produced when the 

 emanation from radium decomposes; and it is not known 

 whether radium, when it gives off its emanation, produces 

 at the same time any other decomposition product. Much 

 has yet to be discovered. Yet it must be acknowledged 

 that a distinct advance has been made, and that at least 

 one so-called element can no longer be regarded as ulti- 



