CONTEMPORARY ADVANCES IN PHYSICS 149 



obtained with so much labor, it will be evident from this article how 

 intricate a process it now proves to be. 



And the incentive for all these labors? At first, the problem seems 

 little more exciting than the tracing of an insignificant leak in an ap- 

 paratus supposedly insulated. There is, however, reason to suspect 

 that this leak is due to causes really sensational. The comprehended 

 part of the ionization of the air being due to known rays such as X-rays, 

 light, and the radiations from radioactive substances, it is natural to 

 ascribe the mysterious residue to rays as yet unknown. It turns out 

 then that these hypothetical rays must lie beyond any yet discovered ; 

 if they are electrons, they must be swifter electrons — if protons, faster 

 protons — if corpuscles of light, then corpuscles of higher frequency and 

 higher energy, than any thus far known; or else they must be particles 

 of a totally new variety. It is not easy to imagine where they could 

 come from on earth, and there are various reasons for supposing that 

 the}' wander in from outer space. Messengers from the depths of space 

 and from the stars — corpuscles of visible light, meteorites, the electrons 

 which are the presumptive causes of the aurora — are constantly being 

 received and ha\e been amply interpreted; it is probable that these 

 cosmic rays have a message of value, perhaps of the first importance. 



The dates of the beginnings of cosmic-ray research have been much 

 disputed, in an unprofitable fashion. I like to set aside the contro- 

 versies by choosing a time extravagantly remote, the year 1785! In 

 that year, it is said. Coulomb made the first acceptable proof that the 

 air of the atmosphere is conductive. It had long been known that an 

 electrified body, mounted on the best available insulator and set up in 

 the air, slowly loses its charge; it had, however, been thought that the 

 "leak" is partly an escape through the insulator or over the surface 

 thereof, partly a carrying-oft' of charge by particles of dust which drift 

 up to the electrified body, touch it and depart — Coulomb found that in 

 addition to these, there is an actual conduction through the air. 



After this discovery, progress ceased for many years. Towards the 

 end of the nineteenth century it was, however, learned that the air 

 becomes much more conductive than normally it is, when rays of 

 certain kinds pervade it — X-rays for instance, and the radiations from 

 radioacti\e atoms. Further, it was found that some part, at least, of 

 the normal conductivity of atmospheric air is due to rays from radio- 

 active atoms blown about by the wind or embedded in the ground. 

 Then came the time for asking: Is there a residual part of the normal 

 conductivity of the air, which cannot be ascribed to rays of any kind 

 hitherto known? With the asking of this question, the way was 

 opened to the sequence of researches, which form the topic of this 



