98 PHYSICS OF MATTER 



even when the terminal from which it comes is acted upon only 

 inductively. The remarkable thing is that the X-ray effects and the 

 luminosity of the tube should then be so great. The unipolar positive 

 discharge in the positive direction and the unipolar negative dis- 

 charge in the negative direction give, in the same time, an X-ray 

 picture of the same intensity, when developed in the same bath, 

 although in the latter case the cathode is only acted on inductively. 



Unquestionably the great problem of to-day is the determination 

 of the nature of positive electricity and its relation to what is left 

 when the /? particles have been removed. When the cathode particles 

 have left the induced cathode terminal it is positively charged, and 

 communicates that charge to the dust in the air, or to neighboring 

 bodies. It does this, however, by a similar inductive action, and the 

 ball discharge traveling over the photographic plate suggests that here 

 also the negative particles are the active ones. 



The few who have search-lights have of late been throwing them 

 upon the great mass of experimental work on the discharge through 

 gases, published during the last generation. It is most instructive 

 to remember that the Crookes tube was known for seventeen years 

 before Roentgen discovered that something was going on outside of 

 it. A repetition of some of the work done on spark discharge, and 

 in particular the work of Wheatstone, in the light of what is now 

 known, would be likely to yield results of the greatest value. It would 

 be of particular value to study by the Wheatstone method the 

 unipolar discharges of the Holtz machine. 



A few words only may be added respecting radioactive phenomena. 



We have long been familiar with the changes in matter, of a char- 

 acter such as may perhaps be described as spontaneous. Many 

 crystals slowly lose their water of crystallization. They give off eman- 

 ations. They explode very slowly. Now emanations, like all other 

 matters and things, have individual peculiarities which enable us to 

 recognize them. The emanation from crystallized sodium carbonate 

 is also given off by all animals and plants, and is evidently a very 

 useful and widely diffused substance. There are many substances 

 which go to pieces and give off energy. They explode. Many of them 

 give off more energy per gram per second than any radioactive body, 

 while radium gives off more energy per gram than any other body. 

 The radium explosion also goes on at a lower temperature than that 

 of any other body. It hardly seems to be warranted, to say that the 

 action is the same at the temperature of freezing hydrogen as at 

 ordinary temperatures, for it does not seem that any high degree of 

 precision has been attained in such a measurement. And certainly 

 it can hardly be claimed that we know what these radioactive bodies 

 would do at a temperature below 16 degrees absolute. 



Seven years ago, an attempt was made in my laboratory to obtain 



