of Glass due to Kathode-Bays. 



121 



Fie 1 . 3. 



a 



to enter suddenly into a vacuum 

 without the necessity for breaking 

 glass or bursting a bladder, a is the 

 glass receiver ; b a ring, between 

 w T hich and the receiver is a sheet of fine 

 tissue-paper, c, on which a number 

 of fragments of glass were placed ; 

 a plate of strong glass situated on top 

 of the ring b admitted of being 

 drawn aside, and thus allowed the 

 air to enter the vacuum, whilst the 

 rest of the apparatus was held fast. 



A luminosity similar to that seen 

 upon previous occasions was ob- 

 served, though less distinct in some 

 respects, perhaps, than before. 



Under precisely similar circum- 

 stances, in the absence of any frag- 

 ments of glass on the tissue-paper, 

 no light was produced, whether or 

 not there were any articles that the 

 air could impinge upon within the 

 receiver. 



A single piece of glass cut into a definite shape was then 

 made to take the place of the great number of fragments, 

 some cotton-wool being placed on the plate of the air-pump 

 in order as far as possible to prevent the piece of glass from 

 breaking. 



The following facts were noticed : — (a) Only a single 

 spark seen. Not as on a previous occasion when a number 

 of fragments of glass were allowed to enter the vacuum, 

 (/3) the spark always appeared on the side of the receiver in 

 the direction in which the plate d had been drawn aside * 

 (see fig. 3) . (7) The piece of glass was found almost vertically 

 under the place where the spark had taken place. 



It seemed probable that the spark was caused by the 

 collision of the piece of glass with the interior of the receiver. 

 Two strong pieces of glass were then struck violently together, 

 and a light similar in every respect to that previously ob- 

 served was produced. 



These experiments seem to justify hypothesis (2) : the 

 dust particles causing the glow by their impacts against the 

 interior of the receiver and against each other ; whilst the 

 brightness of the luminous spots may be attributed to sparks 

 produced by the larger fragments of glass, and to the bom- 

 * It is probable a priori that this should take place, 



