of Glass due to Kathode-Rays. 119 



that produced by the friction of mercury against glass in a 

 barometric tube*. 



In view of testing hypothesis (1) the following experiments 

 were made : — 



A bladder was fastened to the neck of a small receiver, the 

 air in which was then exhausted until a vacuum of about 

 20 millim. was obtained. 



The bladder was then caused to burst, and a stream of 

 yellowish light was seen to descend within. This experiment 

 was repeated several times with receivers of various heights ; 

 and it was found that in the case of tall receivers the light 

 was confined to the higher portion, near the mouth of the 

 receiver. 



The intensity of the light seemed to depend upon the 

 pressure in the vacuum, that is to say, upon the violence 

 with which the air entered the interior of the receiver. 



The general appearance of this yellowish light seemed to 

 indicate that the luminosity in this case was partly, if not 

 altogether, due to parts of the bladder taking fire, being- 

 heated by the friction of the air against them, or by being 

 suddenly torn. Articles of various materials, such as those 

 mentioned by Beccaria, were placed in the interior of the 

 receiver, also a plate of glass supported horizontally so that 

 the air on entering might impinge directly upon it. 



A light in some respects, perhaps, resembling that men- 

 tioned by Beccaria was observed, but only with short receivers; 

 for in the case of tall ones, under precisely similar circum- 

 stances, no light was perceived in the lower region where 

 the articles lay ; thus suggesting that in the former case the 

 portions of burning bladder had been stopped, in their fall, by 

 the articles within the receiver. Beccaria took no precautions 

 to determine the part played by the bladder in the pheno- 

 menon, or rather, he entirely failed to notice that it ever became 

 luminous. 



In order to stop the bits of bladder, gauzes of various 

 materials and different thicknesses were tried ; such as should 

 stop the pieces of bladder but permit the air to rush through 

 without much impedance. The glass bottles beneath the 

 gauze, however, were not seen to give rise to any luminosity. 



A similar plan was made use of when air was allowed to 

 enter a vacuum by the breaking of glass. 



The apparatus employed for this purpose (see f\.g. 2) con- 

 sisted of a cylindrical receiver, A ; a ring, a, which fitted 

 closely into the cylinder and was supported by iron rods 

 * See Armstrong, Phil. Mag*, ser. 3, vol. xviii. (1844), 



