338 Prof. J. Trowbridge on the Spectra of 



7. Conclusions from these Observations. 



We arrive then at the following conclusions : — 



(1) The terminal velocity acquired by a small bubble 



ascending through a viscous fluid is the same as that 

 which would be acquired by a solid sphere of the same 

 size and of density differing from that of the fluid by 

 the same amount, 



(2) When the motion is so slow that no eddies are set up 



in the fluid the velocity of the bubble agrees with that 

 deduced from theoretical considerations by Stokes, on 

 the assumption that no slipping occurs at the boundary. 



(3) When the velocity exceeds a definite critical value, 



the terminal velocity of bubbles and solid spheres is 

 given by the empirical formula 



, — ha 



Y = k( P ~ (r ' 9 )" a 



V 3 



where the value of the constant k is nearly i, and of h 



nearly 



[To be continued.] 



XXXII. The Spectra of Hydrogen and the Spectrum of 

 Aqueous Vapour. By John Trowbridge*. 



IT is customary to consider that there are two spectra 

 characteristic of hydrogen — a four-line spectrum, so- 

 called, and another consisting of many lines widely dis- 

 tributed through the spectrum, known as the white spectrum. 

 The four-line spectrum appears when a condenser-discharge 

 is employed with what is called dry hydrogen ; it is also 

 readily produced in steam and water-vapour. From the fact 

 that a condenser-discharge seems necessary to excite it in dry 

 hydrogen, it is supposed to indicate a higher temperature 

 than the white spectrum. 



The four-line spectrum is found in the atmosphere of the 

 sun ; and is a characteristic spectrum of certain types of stars. 

 There are also other lines attributed to hydrogen in the stars 

 which are supposed to indicate conditions of pressure and 

 temperature which perhaps can be imitated and studied in 

 laboratories. 



I propose to show in this paper that conclusions in regard 

 to temperature and pressure of hydrogen in celestial bodies, 

 deduced from observations on hydrogen enclosed in glass 

 tubes, are untrustworthy ; and that electrical dissociation must 



* Communicated by the Author. 



