183 
sublimation tube (§ 18) does not prevent the transition of one modifi- 
cation of sal-ammoniac to the other one. 
23. From the experiments of $$ 2l and 22 we may conclude, 
that the observed change of sign of the electric double refraction 
cannot be explained by a change of orientation of the particles 
constituting the fog. 
24. It seemed interesting to investigate the behaviour of a fog 
obtained by blowing finely powdered, not very recently sublimated 
sal-ammoniac into the observation tube, the analogon of the experiment 
described in $ 8 with glass and different tartaric acid salts. The 
displacement of the dark band ought to be now upward. We could 
confirm this expectation. 
25. Recently Prof. Vorer has been occupied with Lancrvin’s theory. 
He kindly communicated to us a result, which admits of experimental 
verification '). From the orientation hypothesis Vorer deduces, that 
an absorbing substance must change its power of absorption for 
natural light. 
We have sought for an action of this kind using the sal-ammoniac 
cloud and we think we have discovered it. The nicols and the glass 
bar of our arrangement were removed. Between the lamp and the 
lens. one cr more plates of ground glass were introduced in order 
to diminish the superfluous intensity of the source of light. A dense 
sal-ammoniac fog was blown through the observation tube, the field 
of view becoming of a red hue. Initially the establishing of the field 
gave no change; after interruption of the air current it caused a 
brightening of the field of view, later this became darker under the 
influence of the electric forces. 
The first brightening apparently is due -to the precipatation of 
particles on the condenser plates; if the field is made zero again 
nothing happens. During the later phase very probably an electro- 
optic effect is observed. The field of view changes from pale yellow, 
to more red hues. This effect could be observed again and again when 
the field was put on and off. 
26. In the last part of our investigation we will investigate whether 
1) Since the above ‘was written Vorar’s paper, Ueber elektrische und magnetische 
Doppelbrechung. I. was published in Göttinger Nachrichten 1912. 
