174 PHYSICAL SCIENCE 



within the tubes. By measuring the rates of 

 diffusion of the emanations into other gases, 

 their densities have been determined approxi- 

 mately and found to be of the order of two 

 hundred times that of hydrogen. 



When the emanations come into contact with 

 solid bodies, they cause these bodies themselves 

 to become temporarily radio-active. This radio- 

 activity, which, in some cases, is found to be 

 acquired more readily by negatively electrified 

 surfaces, has been traced to radio-active deposits 

 clinging to the surfaces. Whatever the effective 

 substance may be, it may be treated chemically, 

 and can be dissolved in some acids and regained 

 as a radio-active residue on evaporation. 



All the three types of radiation considered 

 above, and known as a, 1^, and y rays, have one 

 remarkable property which, at first sight, is not 

 shared by the emanations just described. The 

 radio-activity of any element, with regard to the 

 emission of these rays, is independent of the com- 

 pound in which that element is contained. Thus, 

 for a mass containing the same amount of the 

 element radium, the activity of radium chloride 

 is the same as that of radium bromide ; while 

 uranium, the metal, has the same activity as it has 

 when combined chemically in uranium nitrate. 

 Moreover, an alteration in the physical con- 

 ditions, such as temperature, which always largely 

 influence the course of ordinary physical and 

 chemical changes, seems, throughout an extended 

 range, to be entirely without effect on the processes 

 involved in radio-activity. Heating to redness, 

 or exposure to the extreme cold of liquid air or 

 liquid hydrogen, equally leave the activities we 



