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a. A current of air was passed successively through bottles with 
a NH, solution and with a HCI solution. The tubes did not reach 
below the surfaces of the solutions. 
The fog, originating in the HCl bottle, was introduced into the 
basin with the exterior condenser plates (see § 10). It was rather 
difficult to regulate the density, so that the field of view was obscured 
nearly immediately. The fog was partly precipitated after the inter- 
ception of the air current and the dark band (§ 3) became visible; 
the establishment of the field (+ 9000 volts) made the band jump 
upwards. 
In this case we were unable to observe a downward motion of 
the band. 
In the present experiment the rotation of the plane of polarization 
(see §11), ie. the dichroism was very small, so that it was difficult 
at first to determine the sign of the rotation. It proved to be, how- 
ever, the same as the one formerly observed. 
In other experiments with the same kind of fog larger rotations 
were observed. 
6. Dried air was passed over heated, previously sublimated sal- 
ammoniac and then introduced into the basin with exterior con- 
denser plates. 
The air current and the heating of the sal-ammoniac being well 
regulated the throwing on of the electric field caused a downward 
displacement of the band, accompanied with a rotation of the plane 
of polarization. After stoppage of the air current, the band after a 
while exhibited the upward displacement. In some experiments the 
downward displacement could not be observed, and only a rotation 
was seen. This especially happened, if the density of the fog was 
initially very great so that the field of view became dark. After 
partial precipitation of the fog the throwing on of the field caused 
an upward displacement of the dark band. 
20. The results now obtained and those recorded in the former 
parts of this paper clearly point to the existence of two modifica- 
tions of sal-ammoniac, the one which is originated first exhibiting 
a positive, the second modification a negative electric double refraction. 
That we may speak of a “direction” of change of the sal-ammo- 
niac modifications is shown by the fact that the positive double 
refraction is always observed in the first place, and only afterwards 
the negative refraction; we never observed with a given fog first 
an upward and then a downward motion of the band. 
