968 
3. During evaporation in vacuum silver develops a deposit against 
the bulb. With increasing thickness the colour of this deposit passes 
through greenish-yellow, orange, red, violet and blue. 
4. The red, the violet and the blue films are distinctly hetero- 
geneous. They consist of a network of very small ultramicrons. The 
yellow deposit shows a hardly perceptible heterogeneousness and 
approaches in structure the amorphous-vitreous state. 
5. The deposits are not proof against the influence of moist air. 
The colour changes in the direction yellow — red — blue and the 
structure becomes coarser. Heating likewise causes a coarsening of 
structure. 
6. Gold forms — in a similar way as silver — coloured depo- 
sits, which are ultramicroscopically heterogeneous. 
led 
?. Tungsten forms a black deposit, ultramicroscopically it is 
not soluble. 
8. Deposits obtained by cathode-atomizing consist as a rule of 
coarser particles than the evaporation-deposits. 
„Delft, Anorg. and Physic-chemical Eindhoven, Lab. of Philips’ 
Laboratory of the „Technische “Carbon Filament Lamps” 
Hoogeschool’ (Technical University). Works Ltd. 
Physics. — “The virtual displacements of the electro-magnetic and 
of the gravitational field in applications of Hamiton’s variation 
principle” ,By Dr. A. D. Fokker. (Communicated by Prof.LoreNtTz). 
(Communicated in the meeting of January 27, 1917.) 
In some papers on EINsreEiN's theory of gravitation Prof. Lorentz’) 
recently applied Hami.ton’s principle to the deduction of the principal 
equations of this theory from one single variation law. Starting from 
an invariant equation he was able to reach conclusions which again 
were represented by invariant equations. It was however not 
necessary to keep the equations invariant during the whole deduction. 
On the contrary, an artifice, consisting in the choice of a specially 
defined virtual displacement (without taking into consideration the 
conditions of invariancy), proved very useful. 
Now it is possible to let the invariancy exist continually during 
1) H. A. Lorentz, On Hamitton’s principle in Einsteins theory of gravitation, 
Proceedings, Kon. Ak. v. Wet. Amsterdam, XIX, p. 751. Over Einsteins theorie 
der zwaartekracht, 1, ll, Ill, Verslagen, Kon. Ak. v. Wet. XXIV, p. 1389, 1759, 
XXV, p. 468. 
