Chemist?^ and Physics. 251 



SCIENTIFIC INTELLIGENCE. 



I. Chemistry and Physics. 



1. Energy as a Dimensional unit. — In a communication to 

 the Munich Academy, Ostwald has suggested the substitution of 

 the unit of energy for the unit of mass as one of the three funda- 

 mental units of an absolute system ; so that in this case all physi- 

 cal magnitudes would be expressed in units of energy, length and 

 time. The assumption that all such magnitudes can be expressed 

 in units of mass, length and time, appears to him a false one. It 

 is not possible in his opinion, for example, to express in centimeters, 

 grams and seconds the value of the temperature of melting ice, 

 of the chemical affinity between hydrogen and chlorine, or of the 

 electric pressure of a voltaic element. There can be no doubt 

 to-day that energy is the most important quantity in physical 

 science. While mass may be the fundamental quantity in 

 dynamics, energy is the only magnitude which is common to all 

 branches of physics. The mutual convertibility of the various 

 forms of energy is the single bond which unites the theories of 

 heat and of electricity, of chemistry and of dynamics. Starting 

 from the principle of virtual velocities, which is capable, the 

 author thinks, of a more rational expression than that of virtual 

 work, the extension of the energy idea over the entire province 

 of physics and chemistry may be formulated as follows : In order 

 that any system whatever containing forms of energy shall be in 

 equilibrium, the necessary and sufficient condition is that for 

 every displacement of the system consistent with the conditions 

 of its existence, the sum of the quantities of energy which appear 

 and disappear shall be equal to zero. Mass is defined by Ostwald 

 as "the capacity of an object for the energy of motion." The 

 following table shows the changes in the dimensional magnitudes 

 of various units required by the new system : 





Old Units. 



New Units. 



Energy 



\min-z] 



w 



Mass 



[m] 



[el-*t*~\ 



Momentum 



[mlt-i] 



\_el-H 





Force 



[mUr*] 



[el-i 





Surface-tension 



[ml~ 2 \ 



\el'^ 





Pressure 



[ml-H- 2 ] 



[el~ s ' 





Activity 



|>£ 2 r 3 ] 



[er 1 " 





With regard to the electrical magnitudes [/a] the unit of quan- 

 tity and [e] the unit of electromotive force, we have \e} = [£/*] ; and 

 hence the dimensions of [^]=:[e£" 1 .] In the same way the 

 dimensions of the unit of current becomes fee -1 *! -1 ] and of the 

 unit of resistance [er 1 e 2 t.] So that the dimensions of electric units 

 may be represented in one of two ways according as e or /x is 

 made the fundamental unit: — 



Am. Jour. Soi. — Third Series, Vol. XLV, No. 267.— March. 1893. 

 18 



