X-RAYS. 



r.N REGINALD MORTON. M.D. 



Most of the readers of this magazine will remember 

 ho\\-. not only Europe, but the whole of the civilised 

 world, was electrified by the announcement that a 

 certain Professor Roentgen, of \\'urzburg. had 

 discovered a new kind of radiation, or light. 



Up to that time the name of Roentgen, outside 

 his own sphere, was little known ; in a moment 

 one might saj- his name became a household word, 

 ami it is safe to conclude will remain so. probably 

 for many generations to come. 



It is e.xtremely doubtful if any pureh' scientific 

 discovery was ever more quickly followed h\ a 

 world-wide and lasting interest, or put to practical 

 use, incidental!}- creating a ver\- considerable industr^•, 

 than this. 



If a new science — radioactivit\' — was not then 

 discovered, it was at least put upon a basis such as it 

 never had before : it paved the road which led to 

 the discover}- of radium ; brought strong and 

 substantial support to the electron theory, which has 

 since so established itself in all scientific minds that 

 it seems strange to think that onh- so recently we 

 w-ere satisfied w-ith atoms and molecules. 



Leaving radium out of the question, our onl}- 

 means of obtaining the Roentgen ra}-s is by the aid 

 of a high tension electric discharge //; vacuo. Who 

 was the first to experiment with electric discharges 

 in rarefied air or gases is unknow n, but it does seem 

 tolerabh- certain that it was ver}- earl\- in the last 

 centur}-. It was not, however, until about 1858 that 

 Geissler made his first vacuum tubes. These were 

 exhausted to about ■0025mm. of mercur\-, and under 

 the electric discharge appeared to be filled with 

 glowing gas, the colour and character of which varied 

 w-ith the particular kind of gas in the tube before 

 exhaustion was commenced. Man\- of these are 

 extremeh- prett}- and form a familiar object at most 

 scientific entertainments. Son-ie \ears later Crookes 

 began experimenting with tubes which, with the aid 

 of greatly impro\-ed appliances, were exhausted to a 

 much higher degree of vacuum. He obser\-ed with 

 the increasing vacuum there was formed a dark 

 space around the kathode, which increased in extent 

 as the vacuum was increased. Concurrentlv the 

 glowing gas which at first entirely filled the tube 

 gradually disappeared. He also found at this stage 

 new- radiations were given off from the kathode, their 

 direction being at right angles to the surface of 

 emission. These he called " kathode rays," and he 

 ascertained from their behaviour to a magnet that 

 the}- carried a negative charge. Owing to their 

 curious properties he was under the impression that 

 he had discovered a fourth state of matter — " matter 

 in radiation " — but he altered this view later on 

 when he found that the observed phenomena were 



capable of a different explanation. B\- means of a 

 concave kathode he was able to bring these raAS to 

 a focus, and b}- directing them on to some substances, 

 such as fluor spar, calcined shells, and various 

 others, he produced brilliant fluorescence and high 

 temperatures if the action w-ere prolonged. He 

 further observed that when the exhaustion was 

 carried to such an extent that the dark space 

 reached out to the iimer surface of the tube 

 itself, the latter showed a beautiful green fluores- 

 cence, as well as the rise in temperature, and 

 that these effects w-ere intensified if the kathode w-as 

 of conca\-e forni and the wall oi the tube at or near 

 the focal [)oint of the latter. 



There is not the least doubt that the X-ra}s were 

 produced in these experiments, and the question 

 ma}- well be asked how- it came that the\- were not 

 observed h}- Crookes. The answer is \'erv simple. 

 Being invisible to our e}-es the\- would easil\- escape 

 observation in the ordinar\- \va\- : then we must 

 remember that Crookes confined his studies to what 

 was going on inside the tube, and at that time there 

 w-as no evidence of an\- kind to suggest even the 

 possibilit}- of radiations i)assing through the wall of 

 the tube. 



The publication of his famous paper soon started 

 man}- other men experimenting in the same 

 direction, and in ISOi Hertz announced that the 

 kathode ra}s would penetrate thin metallic sheets if 

 jilaced inside the tube, and within their path. Soon 

 after this Hertz unfortunately died, but his experi- 

 ments were carried oi-i b}- his assistant Lenard, who 

 in 1894 had a tube made with an aluminium window-. 

 This, when excited with an induction coil current, 

 allowed the ra\-s to pass outside the tube, and it was 

 obser\-ed that the}- would also pass through other 

 opaque bodies, such as paper, giving rise to fluor- 

 escence and phosphorescence in certain substances, 

 and even affecting the sensitive salts of silver in a 

 similar manner to ordinary light. The only mistake 

 made by Lenard in these epoch-making experiments 

 was in thinking that all these effects were due to the 

 kathode rays alone. 



In November, 1895, Roentgen, while experimenting 

 with a Crookes tube, noticed that a small screen 

 coated with barium platinoc\-anide fluoresced 

 brillianth- in the proximit}-. though the tube was 

 covered w ith cardboard and no light could be seen 

 issuing from it. Following up this phenomenon he 

 made out that this radiation came from a spot on 

 the Crookes tube corresponding to the point of 

 impact of the stream of kathode rays. He further 

 ascertained that all substances were transparent to 

 this new form of radiation, though in var\-ing 

 degrees, depending on the atomic w-eight of the 



494 



