398 Mr. S. H. Freeman on the Question 



This system, in which the velocity- unit is equal to the cri- 

 tical velocity, may be called the critical system of measures ; 

 and here the values corresponding to it of the electric and 

 magnetic units may also be collected : — 



j>J =v. d. [m s ] =(i*\*8- 1 = - weber, 

 [e d ] =v. d. [«J =/J\* = l coulomb, 

 [i rf ] =v. d. [Q =fJ\h- ] = l ampere, 



[E d ]=v.d.[EJ=,AV*=£volt/ 



[R rf ]=v.d.[RJ=X^=^ohm, 

 [C d ]=v.d.[CJ=X- 1 * 3 =|farad. 



Although this critical system of measures is not suited for 

 practical measurements, and especially for the preparation of 

 standards, because the critical velocity is not yet known with 

 sufficient precision, yet it may offer great conveniences in 

 theoretic investigations, on account of the accordance between 

 the values of the static and dynamic units. 



L. On the Question of Electrification by Evaporation. By S. 

 H. Freeman, Felloio in Physics, Johns Hopkins University, 

 Baltimore, Md* 



IT has been very commonly believed that evaporation is an 

 important source of atmospheric electricity. So far as 

 this belief has an experimental basis, it is to be found in the 

 researches of Pouilletf and of Tait and WanklynJ. In com- 

 parison with their work the experiments of Volta, Saussure, 

 and others are of little value. As the result of an elaborate 

 series of experiments, Pouillet came to the following conclu- 

 sions: — 



1. Simple changes of state never give the least sign of elec- 

 trification. 



* Communicated by the Author. 



t Annates cle Chimie etde Physique. 8er. '2. tome xxxv. p, 401, and xxxvi, 

 p. 4. 



\ Phil. Mag. ser. 4. vol. xxiii. p. 40J. 



