H. V. Gill — Electric Discharge in Geissler Tithes. 407 



Another proof is afforded by a fact which seems to have met 

 with no explanation. Sometimes it was noticed that the interior 

 of the tube was covered by rings of a dark deposit; these i 

 have been proved by several to be proper to the dark spa 

 For instance, Gassiot* states : "In one of Cetti's tubes originally 

 charged with arseniuretted hydrogen there was such a deposit. 

 ..." This result (i. e. of experiments which we need not dwell 

 on), appeared to me to explain that the deposit in Geissler 

 tubes did not arise from particles of the negative electrode, bat 

 from the gas with which it was originally charged ; their being 

 deposited exactly in the dark portions between the luminous 

 disks may lead to a correct explanation of a phenomenon which 

 has hitherto baffled the ingenuity of experimentalists." 



Now our theory seems to supply an explanation of this fact. 

 Let us remark that these rings have not been noticed in the 

 case of pure gases such as hydrogen, oxygen, carbonic acid gas, 

 etc. In the case referred to by de la Rue and Miiller the 

 nature of the gas is not stated. Probably the latter refer to 

 the same tube as does Gassiot, for they say it was noticed in a 

 tube which had belonged to Gassiot. To proceed, the gas is 

 thrown into a series of stationary waves, giving rise to nodes 

 and loops. At a node the gas is alternately in a state of com- 

 pression and rarefaction. When the gas is in a state of com- 

 pression the discharge takes place between the molecules. 

 This produces a luminous ring which is a stratum. Now, 

 although the mean temperature of a stratum as measured by 

 any form of thermometer is but slightly higherf than that at 

 a dark space, there is no doubt but that the heat generated 

 between the molecules is very great. This heat decomposes 

 the AsH 8 Now when the compression changes into a rarefac- 

 tion part of the gas thus decomposed makes a slight excursion 

 towards the loop or dark space. The As which has been lib- 

 erated at the strata is condensed on the sides of the tube in 

 this cooler atmosphere and thus forms the rings. In the elab- 

 orate experiments of Gassiot, de la Rue, etc., the dischargee 

 were very regular, and hence the strata, as appears from 

 eral passagesin their memoirs, occurred at the same place. _ So 

 that there is no difficulty in conceiving how the dark pings 

 always coincided with the obscure places. It would be, in 

 fact,' a form of Marsh's arsenic test. We do not knew if the 

 deposit in such a tube has been subjected to the arsenic test. 

 This would be an interesting research. In Marsh's test, it will 

 be remembered, the arsenic mirror is formed on that part of 

 the tube just beyond the portion which has beeo heated. 



From this last paragraph appears at once how the wry 



* Report Brit. Assoc. (Sect.), p. 4G. 18G9. 



f Cf. R W. Wood, Physical Review. Xov.-lVi\. 



