884 
If the internal equilibrium in the metal sets in less rapidly than 
the metal goes into solution, the internal equilibrium in the iron- 
surface will be disturbed in the noble direction, i.e. the surface will 
become richer in ferri-ions, and poorer in electrons. 
This case actually presents itself here, and the said disturbance 
increases with quicker stirring and also through increase of the ferri-ion 
concentration, and fail of temperature. Owing to the negative catalytic 
influence of the nitrate ion, the iron could easily be passified in 
this way. 
It coutd be established with certainty that not the hydrolytically 
split off nitric acid effects this disturbance, for even a nitric acid 
solution of 32°/, by weight of NHO, was not able to make the 
iron passive. 
Amsterdam, Dec. 16, 1916. Anorg. Chem. Laboratory of 
the University. 
Physics. — Einstein's theory of gravitation and Here.orz’s mecha- 
nics of continua’. By GouNNar NorDsTRÖM. (Communicated by 
Prof. H. A. Lorentz). 
(Communicated in the meeting of November 25, 1916). 
In a way somewhat different from that used by Lorentz and 
Hinpert, Erstein has recently deduced his gravitation theory from 
Hamiuron’s principle '). In doing this he divides Hamrron’s function 
$* into two parts: 
DEE JAE st i EN 
in this way that the first part G* depends on the g”’s only and 
their first derivatives with respect to the coordinates g”” while the 
second part Mr contains the g””s only and besides certain variables 
q(e), Which determine the state, and the derivatives of these q(o)’s. 
By varying the g”’s Einstein obtains the equations for the gra- 
vitation field in the form 
= (ee ne 
z Org Og?” Og? Og?” 
1) A. Ernsrein. Hamitronsches Prinzip und allgemeine Relativitätstheorie, Ber- 
liner Sitzungsberichte 1916. p. 1111. 
