427 
Above these temperatures they crystallize into cubes, below them 
into the well-known skeleton form. If they are formed at ordinary 
temperature and undergo later a transformation, there are first 
formed cubes for them both (the ¢-modification, unstable at ordinary 
temperature). Later on a transformation into needles occurs (the 
a- or stable modification). Ammonium-iodide shows other pro- 
perties; at ordinary temperature it crystallizes into cubes. SCHEFFER 
has proved however, that NH,I is also dimorphous and enantiotropiec, 
but that its point of transition lies near —16°. He had the kindness 
to tell me this result, which has not yet been published. At ordinary 
temperature NH,I will therefore first exist as needles and then as 
cubes and from what has been said above, we might expect that 
the change of sign of the double refraction will have the opposite 
direction for NH,I ‘as for NH,Cl and NH,Br. This not being the 
case, we may conclude that the allotropy is of no influence in this 
question. I express still my thanks to Dr. Scuerrer, who was so 
kind as to test the purity of the NHL. 
7. Some remarks on the explanation of these phenomena. In my 
opinion the explanation may not be sought in the direction of the 
electro-optic Kurr-effect. Firstly not because for ammonium-chloride 
the double refraction approaches a maximal value, while the Kerr- 
effect always shows a quadratic dependence on the field-and further 
because the small density of the cloud (generally less than 0,00005) 
would oblige us to ascribe to ammonium-chloride a constant of 
Kerr of the order of 10° X that of CS,, which would be very im- 
probable. I tried to find another explanation by assuming the par- 
ticles to be directed by the field. Ammoniumehloride being regular in 
both modifications they should have then a stretched form. Mieroscopi- 
cally however such a form could not be detected. Perhaps the 
particles were too small (5.10-5 em. and smaller). Nor was an 
orientation observed microscopically. Besides the vivid BROWNIAN 
movement of the particles of the ammonium-chloride many of them 
were seen — in the case of an aiternating field — to get into 
oscillation, as had formerly already been observed by Corron and 
Mouton ') and recently again by KRruyr *) in some colloidal solutions. 
Fall experiments with particles of ammoniumchloride have made it 
probable however that an electric field causes an orientation 
1) Corron et Mouron, Les ultramicroscopes. Les objets ultramicroscopiques. 
Paris 1906, pag. 154 and foll. 
2) Kruyt, these Proceedings XVIII, p. 1625. 
