1892.] On the Velocity of Crookes Cathode Stream. 331 



to appear, however, will be the 

 longest low- temper at u re lines of 

 the various chemical elements. 



Stage 2. The hydrogen lines 

 will continue to thin out, and the 

 spectra will show many more of 

 the high-temperature lines of 

 different elements. These will 

 differ from the lines seen in stars 

 of increasing temperature owing 

 fco the different percentage corn- 

 position of the absorbing layers, 

 so far as the known lines are con- 

 cerned. 



Stage 3. With the further 

 thinning out of the hydrogen 

 lines and reduction of tempera- 

 ture of the atmosphere, the ab- 

 sorption flutingsof the compounds 

 of carbon should come in. 



that its spectrum shows many of 

 the longest lines of iron. 



The conditions at this stage of 

 cooling are satisfied by such stars 

 as /3 Arietis and a, Persei. In 

 the spectrum of these stars 

 nearly all the solar lines are 

 found, in addition to fairly broad 

 lines of hydrogen. 



There is undoubted evidence 

 of the presence of carbon absorp- 

 tion in the solar spectrum and 

 the spectrum of Arcturus, the 

 only star which has yet been in- 

 vestigated with special reference 

 to this point. 



The photographs, then, give us the same results as the one 

 formerly obtained from the eye observations. 



Comparison is then made between the groups in the classification 

 first suggested by the eye observations, and the various sub-divisions 

 in which the photographs have been arranged. 



II. " On the Velocity of Crooked Cathode Stream." By LORD 

 KELVIN, P.R.S. Received December 3, 1892. 



In connection with his splendid discovery of the cathode stream 

 (stream from the cathode in exhausted glass vessels subjected to 

 electric force), Crookes found that when the whole of the stream, or 

 a large part of the whole, is so directed as to fall on 2 or 3 sq. cm. of 

 the containing vessel, this part of the glass becomes rapidly heated 

 up to many degrees, as much as 200 or 300 sometimes, above the 

 temperature of the surroundings. 



Let v be the velocity, in centimetres per second, of the cathode 

 stream, and /> the quantity of matter of all the molecules in 1 c.c. of 

 it. Supposing what Crookes' experiments seem to prove to be not 

 far from the truth, that their impact on the glass is like that of in- 

 elastic bodies, and that it spends all their translational energy in 

 heating the glass. The energy thus spent, per square centimetre 



