PROPERTIES OF CORPUSCLES. 73 



CORPUSCLES AND X-RAYS. 



When the air pressure in the bulb producing cathode 

 rays is so low that the glass of the bulb becomes brilliantly 

 phosphorescent, an entirely different set of rays proceeds 

 from the tube, having properties that differentiate them 

 altogether from the cathode rays, or corpuscles, that pro- 

 duce them by their impact on the glass or on a metal 

 plate within the bulb. These rays illuminate a phospho- 

 rescent screen, affect a photographic plate, and have an 

 astonishing power of penetrating substances opaque to or- 

 dinary light. They are called X-rays because at the time 

 of their discovery their nature was an unknown quantity. 

 And so it is, to a certain extent, even now. They are not 

 corpuscles; though it is not one of the least interesting 

 properties of corpuscles that they produce X-rays. Their 

 probable nature, and their relation to corpuscles, will be 

 considered when we wish to make use of them in the 

 elucidation of another problem. 



CORPUSCULAR CONDENSATIONS. 



Except in the phenomena of radio-activity, which con- 

 stitutes the subject-matter of the next part of our work, 

 these ultra-atomic particles, or corpuscles, are found only 

 in gases or metals at high temperatures or in the low pres- 

 sure which exists in a good vacuum. The reason for this is 

 that no sooner is a corpuscle let loose than it tends to unite 

 itself with anything else available, whether this be another 

 corpuscle, an atom or a molecule; so that it is only in the 

 long, open spaces of a good vacuum, where they have a 

 path free from much chance of collision, that we can study 

 them as they are. At the ordinary pressure of the atmos- 

 phere, or at ordinary temperatures, they are no sooner set 



