178 
If in this expression we substitute the values A=—=1,985, t= 21,2, 
and r == 0.15, which two last refer to the boiling point, we find for 
the heat of evaporation at the boiling-point: 
9200 calories. 
The experimental determinations of BrerrumLOT and OrEr *) appre- 
ciably differ from this value. From a number of values which differ 
pretty considerably from each other, which, however, all of them 
lie lower than the above mentioned one, they consider 8600 calories 
the most probable. We, however, think that we have to prefer our 
calculation, the more so as the determinations which have served 
for our calculation, just lie in the temperature region over which 
Ramsay and Youne’s investigation extended, and the determinations 
of the latter do not practically differ from ours. 
In conclusion we wish to avail ourselves of this opportunity to 
express our thanks to Prof. Smits for his advice in the experimental 
difficulties experienced by us, and for the interest shown by him in 
our work. 
Anorg. Chem. Laboratory of the 
University of Amsterdam. 
Physics. — “Electric double refraction in some artificial clouds and 
vapours.” (Third part). By Prof. P. Zeeman and C. M. HooerNBoom. 
18. The results obtained with the sal-ammoniae fog might be 
explained by postulating the existence of two varieties of sal-ammoniac 
erystals. This hypothesis was put forward in § 17. In the textbooks 
on crystallography, which were at the disposition of the authors, 
nothing however, relating to dimorphism of sal-ammoniac could be 
found. This seemed rather unfavourable to the proposed explanation. 
We are much indebted therefore to Dr. F. E. C. Scurerrer, who 
gave us some references to the chemical-crystallographical literature, 
from which it appears that the dimorphism of sal-ammoniac is a 
well-known fact (see v. Groty, Chemische Kristallographie. Band I. 
S. 167. 1906). 
Stas *) while sublimating NH,Cl had observed a phenomenon closely 
resembling the transformation of polymorphous substances; he did 
not try however an explanation and it seems that he did not think 
of dimorphism. 
1) Ann. de Ch. et de Ph. (5) 30 398 (1883). 
2) Spas. Untersuchungen über die chemischen Proportionen u.s.w. deutsch von 
Aronstein. S. 54. Leipzig 1867. 
