370 Prof. J. Trowbridge on the Spectra of 



Similar experiments were made with Rochelle salt, and the 

 spirals observed with a plate 1'75 cm. thick. Monochromatic 

 light was found to be essential. The rotation was 12 deg. 

 per cm. to the right. The value calculated from the specific 

 rotation of a 20-per-cent. solution is 4 deg. per cm. in the 

 same sense. On account of the symmetry of the crystal no 

 difference between the two axes is to be expected. 



XXXVII. The Spectra of Hydrogen, and some of its 

 Compounds. By Prof. John Trowbridge *. 



[Plate VI.] 



IN a late paper f I expressed the conviction that the so- 

 called line-spectrum of hydrogen cannot be considered 

 apart from the spectrum of water-vapour ; and that one can 

 never be sure that one is observing, with a condenser discharge, 

 a pure spectrum of hydrogen. I am convinced from further 

 experimentation, that this conclusion is correct; and I am 

 also led to the conclusion that a certain amount of water- 

 vapour is essential in all electrical discharges through gases. 



Just as aqueous vapour seems to play an important role in 

 most chemical reactions, so, it seems to me, its presence in 

 rarefied gases, contained in ordinary glass tubes, enables a 

 dissociation to take place which determines the strength and 

 character of the electrical discharges. 



I am led. moreover, to the conclusion that pure hydrogen 

 is a perfect insulator ; and that the passage of electricity 

 through a gas depends upon the dissociation of the hydrogen 

 and oxygen, by means of which change in the distribution of 

 energy the gases are made luminous. Before proceeding 

 to an account of my experiments, I will state some of the 

 grounds upon which I base my belief that pure hydrogen is 

 an insulator of electricity. 



V. Schuman, in an important paper J, has shown that a 

 column of pure hydrogen at atmospheric pressure transmits 

 the ultra-violet rays as well as the most perfect vacuum he 

 has been able to obtain. Now Maxwell's electromagnetic 

 theory of light demands that the space between us and the 

 sun, or, in other words, the vacuum of space, should be a 

 perfect insulator, otherwise the electromagnetic waves would 

 be completely absorbed, and the earth would remain in 

 darkness. This observation of Schuman seems to me one 



* Communicated bv the Author. 

 t Phil. Map-. Sept/1900, 

 t Ann, der Physik, 1901. 



