746 THE POPULAR SCIENCE MONTHLY. 



measurement necessary to the completion of the system by taking ad- 

 vantage of Faraday's law, that the quantity of metal decomposed in 

 an electrolytic cell is proportional to the Avhole quantity of electricity 

 that passes. The best metal for the purpose is silver, deposited from 

 a solution of the nitrate or of the chlorate. The results recently ob- 

 tained by Professor Kohlrausch and by myself are in very good agree- 

 ment, and the conclusion that one ampere flowing for one hour decom- 

 poses 4"025 grains of silver, can hardly be in error by more than a 

 thousandth part. This number being known, the silver voltameter 

 gives a ready and very accurate method of measuring currents of in- 

 tensity, varying from ^^ ampere to four or five amperes. 



The beautiful and mysterious phenomena attending the discharge 

 of electricity in nearly vacuous spaces have been investigated, and in 

 some degree explained, by De La Rue, Crookes, Schuster, 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 phosphorescent spectrum of certain 

 earths, and, after encountering difficulties and anomalies of a most be- 

 wildering 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 per- 

 severance of which it is the final reward. 



A remarkable observation by Hall, of Baltimore, from which it ap- 

 peared that the flow of electricity in a conducting sheet was disturbed 

 by magnetic force, has been the subject of much discussion. Mr. Shel- 

 ford 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 gen- 

 eration of scientific men has been the establishment and application of 

 the great laws of thermo-dynamics, or, as it is often called, the me- 

 chanical 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 mechanical 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 begin- 

 ning 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 themselves readily to the demands that are made upon 



