426 
We see, that the dispersion may be very strong and that it is 
such that the rays with the greatest index of refraction show the 
smallest double refraction, which result is in contradiction with all 
that has been found till now on the electro-optic Kerr-effect. Still 
it may be mentioned, that for the same mean double refraction the 
cloud showed a greater dispersion when it had stood for some time 
than when it had just been formed. 
With the «-modijication no detinite result was reached. If however 
dispersion is shown by this too, then the rays of the greatest refrac- 
tivity will also show the greatest double refraction. 
6. Kuperiments with other clouds. Results similar to those found 
for ammonium chloride were reached with clouds of indigotine and 
ammonium-bromide and -iodide. 
A remark of Brocu ') on the properties of a cloud of indigotine 
in a magnetic field induced me_to an investigation of this cloud. 
It was produced by heating the solid substance. For many experi- 
ments indigotine pur. cryst. of the “Pharm. Handelsvereeniging” was 
used. This heating however always caused decomposition, accom- 
panied by formation of clouds’). A dry air current led over the 
heated substance carried the mixture to a condensator vessel, where 
it was proved to become double refracting and sometimes slightly 
dichroistic under the influence of an electric force, while the light 
was dispersed stronger in a perpendicular direction. 
It has not yet been found out which substance it is that shows 
these properties. For a cloud of aniline, one of the products of 
decomposition, not any electro-optic effect could be detected. 
Ammonium-bromide and -iodide showed just the same properties 
in the electric field as ammonium-chloride, aiso in this respect, that 
one and the same cloud, which at its formation was positively double 
refracting, after some time had become negative. If this change of sign 
is connected with the allotropy of those substances, this behaviour 
might have been expected for ammonium-bromide. For, as to their 
properties NH,Cl and NH,br are very similar: they are enantiotropic, 
as has been proved by WaLrace®); for the points of transition 
SCHEFFER found: for NH,CI184°,5*) and for NH,Br about 137°®). 
1) Brocu, Loc. cit. 
2) Thus Broen speaks of “fumées provenantes de la sublimation de l’indigotine”’. 
3) Warrace, Centralblatt fiir Mineralogie u.s.w. 1910 S. 33. 
4) Scuerrer, These proceedings, XVIII p. 446 and 1498. 
5) Scuerrer, Handelingen 15de Ned. Natuur- en Geneesk. Congres te Amsterdam, 
pag. 242, Haarlem 1915. 
