of Glass due to Kathode-Rays. 117 



sufficiently sensitive to the faintest light, broke one of the 

 larger of these bulbs with a hammer, at the same time pro- 

 tecting his face by a plate of thick glass. A faint white glow, 

 which lasted but for a very small fraction of a second, was 

 seen in the place where it was known the bulb had lain. The 

 experiment was repeated in the presence of several persons, 

 who testified to the same effect. The intensity of the light 

 seemed to depend upon the size of the bulb — the larger bulbs, 

 about 7 or 8 centim. in diameter, giving a greater intensity 

 than the smaller ones ; whilst with bulbs of not more than 

 2 centim. diameter no luminous effect was perceived. 



A circular plate of glass, about 7 centim. in diameter and 2 

 millim . thickness, was then placed on the mouth of a receiver, 

 the opening of which was about 5 centim. in diameter, and 

 the contact made air-tight, whilst the air in the receiver was 

 exhausted until a vacuum of about 20 millim. had been 

 obtained. 



The plate of glass was then broken with a hammer. A 

 beautiful stream of white light was visible throughout the 

 whole receiver. The light, which lasted for a small fraction 

 of a second, resembled that produced when vacuum-bulbs 

 were broken, though it was more distinct, and consisted not 

 merely of a uniform glow, as in that case, but was marked, in 

 addition, by a number of luminous spots of various sizes : in 

 fact the effect presented the appearance of a faint nebula. 



On account of the extreme faintness of the light, it was 

 found impossible to photograph it : at least any attempts in 

 this direction have hitherto proved unsatisfactory, even with 

 the most sensitive plates I could procure. 



In some instances the luminous spots were observed to be 

 unusually large ; the fragments of glass were in such cases 

 also found after each experiment to be of an exceptional size, 

 often 7 or 8 square centim. in area. 



This fact is especially worthy of notice, as it undoubtedly 

 indicates that the fragments of glass themselves had become 

 luminous, the continuous glow in all probability having been 

 caused by the very minute fragments. 



Different kinds of glass were used without giving any per- 

 ceptible difference in the effects. 



Thin brittle plates of cast-iron and steel were tried in the 

 place of glass, but yielded negative results, owing, perhaps, 

 to the fact that they did not break up into innumerable 

 fragments as glass did readily when struck with a hammer, 

 or because the air was not consequently permitted to enter 

 the vacuum very suddenly and with sufficient violence. 



