58 BELL SYSTEM TECHNICAL JOURNAL 



Already in expressing these restrictions, certain principles of radio- 

 activity have been implicitly assumed; it is necessary to state them 

 explicitly. 



In the first place, I have spoken of the radioactivity of the elements 

 alone; this is permissible, because radioactivity is definitely a property 

 of individual elements. This does not mean merely that radioactivity 

 is a property of a limited number of elements in certain states and a 

 limited number of compounds of these and other elements, as seems to 

 be true of ferromagnetism. It means that wherever there is a particu- 

 lar radioactive element, free or compounded, gaseous or liquid or solid, 

 the characteristic rays of that element are emitted in a degree propor- 

 tional to the amount of the element and not affected in the least by its 

 condition or its state of combination. A given amount of radium emits 

 the same kinds of rays at the same rate whether it is a piece of pure 

 metal, or is combined with chlorine in radium chloride, or with sulphur 

 and oxygen in radium sulphate. A given amount of radon emits 

 rays of the same sort at the same rate whether it is gaseous as at normal 

 temperatures, or frozen by submerging its enclosing tube in liquid air. 

 Samples of some of the radioelements have passed through combination 

 after combination in the chemical laboratory, being released from one 

 compound only to enter into another; their activity was meanwhile 

 being measured by the most delicate available tests, but it was never 

 found to be affected in any perceptible degree. There is no other 

 property of an element, excepting mass, of which this can be said with- 

 out reservation.^ 



The indifference of radioactivity to the state of combination of the 

 elements which display it extends also to all their other circumstances. 

 In modern laboratories it is feasible to subject pieces of matter to very 

 powerful, severe and violent agencies; heat enough to melt any 

 element, cold enough to freeze any substance, electric fieldstrength 

 high enough to tear electrons out, high magnetic fields, intense illumi- 

 nation, bombardment by multitudes of fast moving charged particles — 

 and all of these have been tried to some extent, some to the utmost 

 humanly possible extent, upon radioactive elements; but in every 

 instance the radioactivity has remained constant without detectable 

 variation, inaccessible and immune to all the powers within human 

 control or under human observation.^ 



^ It can be said of the higher-frequency emission-lines and absorption-edges of the 

 X-ray spectra of the elements, but not unreservedly; for since the lower-frequency 

 lines and edges of an element do vary slightly but perceptibly when its state of com- 

 bination is altered, there is a strong presumption that the higher-frequency spectra 

 will likewise be found to vary as soon as the accuracy of the measurements is increased 

 say five- or tenfold. 



^ Influences of sunlight upon radioactivity are reported now and then in the 

 Comptes Rendus; but it seems exceedingly unlikely that something immune to every 

 other known agency should be susceptible to this particular one. 



