1896.] Argon and Helium submitted to Electric Discharge. 269 



helium and argon. Also that a large proportion of helium must be 

 added to argon to become visible ; while a minute trace of argon is 

 visible in helium. And combining these results with those relating 

 to the spark-gap or to the continuous discharge, it would appear that 

 at atmospheric pressure, a discharge passes much more readily 

 through helium than through the other gases, while at very low 

 pressures, if conveyance of current can be inferred from luminosity 

 of spectrum, the current passes more readily through all other gases 

 than through helium ; and that a current is conveyed more readily 

 by nitrogen than by argon. 



In the present state of our ignorance on what occasions the passage 

 of current through gases, it is perhaps premature to speculate. But, 

 accepting the hypothesis which appears to be gaining ground, namely, 

 that a species of electrolysis takes place during the passage — a tenta- 

 tive explanation may be put forward. It may be supposed, as Hr. 

 Lang has pointed out (" Dielectricitatsconstaute," ' Wied. Ann.,' 56, 

 p. 543) that a gas, such as hydrogen, at atmospheric pressure, is 

 partially, but very slightly, ionised, or perhaps better, polarised, if 

 that expression refers to atoms capable of receiving and transmitting 

 an electrical charge. 



Now, the conveyance of the current is conditioned by at least two 

 circumstances — first, by the number of such atoms per unit volume ; 

 and second, by their relative velocity, which may be taken as propor- 

 tional to the square root of the density of the gas. Hence, it might 

 be inferred that if only few atoms are present, great resistance will be 

 offered — first, because of their fewness ; second, because of the fric- 

 tional resistance offered by the molecules of gas, as distinct from the 

 atoms. It is not so easy to understand why a high electromotive 

 force is required to overcome this resistance, unless it be supposed 

 that the atoms capable of conveying charges neutralise each other by 

 combination before the charge can be transmitted from one electrode 

 to the other. Waiving this difficulty for the present, the hypothesis 

 agrees with the known fact, that decrease of pressure promotes dis- 

 sociation, where the number of ions or molecules produced by dis- 

 sociation is greater than the original number of undissociated 

 molecules ; and with increased dissociation there should be, as there 

 appears to be, decreased resistance in the case of diatomic gases, such 

 as hydrogen, oxygen, and nitrogen. 



In considering gases, presumably monatomic, such as argon and 

 helium, we see that the former offers considerably less resistance to 

 the discharges at atmospheric pressure than the diatomic gases ; and 

 the latter very much less. These peculiarities may be due to their 

 monatomic character, and the great difference between the length of 

 the spark-gap in argon and in helium may be dependent on their 

 difference of densities, and be connected with the length of the mean 



