1891.] liquid electrodes in vacuum tubes. 223 



(A), (B) and (C) are regarded as forming the negative light 

 or discharge, and (D) as separating the negative and positive 

 discharges. (A) is very inconspicuous, if actually existent, at 

 gaseous pressures exceeding 1 or 2 mm. of mercury, but at very 

 low pressures it is fairly bright though very thin. 



(B) is also insignificant so long as the pressure exceeds a few 

 mm. of mercury, but at very low exhaustions it has been ob- 

 served to exceed a length of 2 cm. Its length has been shown 

 not to depend much on the material of the cathode when metallic. 

 It is also usually but little dependent on the strength of the 

 current. It varies to a considerable extent with the nature 

 of the gas, being according to Professor Crookes 1 decidedly longer 

 in hydrogen and shorter in carbonic acid gas than in air. In 

 any one gas it is supposed to increase in length as the pressure 

 is reduced, so that its magnitude gives a useful if not very exact 

 indication of the degree of exhaustion 2 . The following table 

 gives some of the measurements of Crookes 3 — altered to mm. — 

 and Puluj 4 for air vacua. 



For a given length of (B) the pressures found by Crookes 

 are very considerably less than those found by Puluj, a dis- 

 crepancy ascribable perhaps to differences between their tubes 

 and cathodes but due probably in greater measure to the un- 

 certainties attending the determination of such low pressures. 



The conditions of Puluj's experiments resembled more closely 

 those of the present paper than did Crookes', so the former's 

 results seem a priori the best for comparison with those de- 

 scribed here. It must, however, be remembered that at the 

 lowest pressures in my experiments, the gas in the tube was 

 doubtless in great measure, if not almost exclusively, vapour 

 from the liquid electrodes. Though not absolutely black, (B) in 



1 Phil. Trans. 1879, pp. 138—9. 



2 Phil. Trans. 1879, p. 137. 



3 Phil. Trans. 1879, pp. 158—9. 



4 Sitzungsberichte Math. Nat. Classe der k. Akad., Bd. lxxxi., Abth. n. Wien, 

 1880, p. 874. 



