194 Professor Silvanus P. Thompson [May 8, 



beyond the suggestion of atomic weight. The nearest approach to a 

 law that 1 have been able to get at yet, on comparing tables of statistics, 

 is that the transparency is proportional to the specific heat. For 

 homologous series this is, of course, the same as saying that the 

 transparency is inversely proportional to the molecular weight. 



Eoentgen found all the heavy metals to be remarkably opaque, 

 while light metals like sodium and aluminium, and even zinc, are 

 remarkable for their transparency. Aluminium, which is opaque to 

 every known kind of light, is transparent, even in sheets halt an inch 

 thick, to these rays. Lithium, the lightest of solid metals, and with 

 an atomic weight 7 as against aluminium 27, is so transparent that I 

 have not been able yet even to see its shadow. Of all liquids water 

 is the most transparent, and it has the highest specific heat of all of 

 them. 



Roentgen further found these rays to be incapable either of refrac- 

 tion by lens or prism,* or of reflection by any polished mirror. 

 Eeflection there is in one sense, that of ditifuse reflection, such as 

 white paper exercises on common light. No lens can concentrate 

 these rays : they are also apparently incapable of being polarised. 

 One difficulty in experimenting on these strange properties is that air 

 itself acts as a turbid medium, reflecting back diffusely, as a smoky 

 cloud would do for ordinary light, a portion of the rays. 



Finding that these radiations differed in so many ways from 

 ordinary light, and while resembling and even surpassing ultra-violet 

 rays in their strong actinic properties, yet differed entirely from them 

 in respect of the properties of refraction, reflection and polarisation, he 

 named them " X-rays." To judge by his own writing, he appeared to 

 wish that they might prove to be longitudinal vibrations in the ether, 

 the possibility of the existence of which has been a subject of specu- 

 lation on the part of some of the most learned of mathematical 

 physicists. Others have suggested that these X-rays are transverse 

 vibrations of a much higher frequency and shorter wave length than 

 any known kind of ultra-violet light. Others again see in them 

 evidence that radiant matter (i.e. kathodic streams of particles) can 

 traverse the glass of a Crookes tube, and regard them as material in 

 their nature. Lastly, it has been suggested that they may be neither 

 waves nor streams of matter, but vortex motions in the ether. 



To follow out the bearings of these speculations, as well as to 

 trace the development of discovery, let us go back a little and consider 

 what was the starting point of Roentgen's research. He was using a 

 Crookes tube. It is one of the difficulties of my task to-night that 

 I have to speak in the presence of him who is the master of us all in 



* Perrin in Paris, and Winkelmann in Jena, have independently found what 

 they believe to be evidence of refraction through an aluminium prism. Both 

 observers detected a slight deviation, but in a direction toward the refracting 

 angle, showing aluminium to have for these rays a refractive index slightly less, 

 with respect to air, than unity. 



