Rayleigh's Address to the British Association. 97 



discharge of electricity in nearly vacuous spaces have been investi- 

 gated and in some degree explained by De la Rue, Crookes, Shuster, 

 Moulton, and the lamented Spottiswoode, as well as by various 

 able foreign experimenters. In a recent research Crookes has 

 sought the origin of a bright citron-colored band in the phosphor- 

 escent spectrum of certain earths, and, after encountering difficul- 

 ties and anomalies of a most bewildering kind, has succeeded in 

 proving that it is due to yttrium, an element much more widely 

 distributed than had been supposed. A conclusion like this is 

 stated in a few words, but those only who have undergone similar 

 experience are likely to appreciate the skill and perseverance of 

 which it is the final reward. 



A remarkable observation by Hall of Baltimore, from which it 

 appeared that the flow of electricity in a conducting sheet was 

 disturbed by magnetic force, has been the subject of much dis- 

 cussion. Mr. Shelford Bidwell has brought forward experiments 

 tending to prove that the effect is of a secondary character, due in 

 the first instance to the mechanical force operating upon the 

 conductor of an electric current, when situated in a powerful 

 magnetic field. Mr. Bidwell's view agrees, in the main, with Mr. 

 Hall's division of the metals into two groups according to the 

 direction of the effect. 



Without doubt the most important achievement of the older 

 generation of scientific men has been the establishment and 

 application of the great laws of Thermo-dynamics, or, as it is 

 often called, the Mechanical Theory of Heat. The first law, which 

 asserts that heat and mechanical work can be transformed one 

 into the other at a certain fixed rate, is now well understood by 

 every student of physics, and the number expressing the mechan- 

 ical equivalent of heat resulting from the experiments of Joule, 

 has been confirmed by the researches of others, and especially of 

 .Rowland. But the second law, which practically is even more 

 important than the first, is only now beginning to receive the full 

 appreciation due to it. One reason of this may be found in a not 

 unnatural confusion of ideas, Words do not always lend them- 

 selves readily to the demands that are made upon them by a 

 growing science, and I think that the almost unavoidable use of 

 the word "equivalent" in the statement of the first law is partly 

 responsible for the little attention that is given to the second. 



