( 281 ) 
A second proof of the isomerism of the two oxides may be given 
by means of measurements of the temperature coefficient of the 
mercuric oxide cell, I shall shortly make a communication on this 
point. 
Summary of the resuits of the investigation. 
1. There is a difference in the free energy of red and yellow 
mercuric oxides (0.685 millivolts at 25°). Contrary to Ost- 
WALD’s opinion, therefore, the substances are isomeric and 
not identical. 
bo 
The measurements of GLAZEBROOK and SKINNER, according 
to which a difference of 7 millivolts at 12° exists between 
the E.M.F.’s of Gouvy standard cells made with the two 
oxides, are incorrect. The difference at 12° C. is less than 
0.6 millivolt. 
Amsterdam, University Chemical Laboratory. 
November, 1899. 
Chemistry. — “The Enantiotropy of Tin” (III). By Dr. Ernst 
ConHeN. (Communicated by Prof. H. W. BAKHUIS ROOZEBOOM.) 
1. In continuation of the investigation of the two modifications 
of tin a large block of Banca tin, weighing about 1/, kilo, was 
inoculated with grey tin, and subsequently submitted to a tempera- 
ture of — 5° C. for 3 weeks in contact with a solution of pink 
salt in order to observe the phenomenon on a larger scale. 
The accompanying figure, from a photograph, shows that the whole 
surface is covered with grey warts, between which the original silver- 
white colour of the metal is to be seen only in a few places. 
Once the change has begun the waste goes forward at the ordinary 
temperature. 
2. It appears from the following communication, which I owe 
to Dr. VAN DER Praars of Utrecht, that the phenomenon of the 
spontaneous change of white tin into the grey modification has oc- 
curred in this country also. In 1883 it was found that the water 
in the water mains of Utrecht contained lead. Since that time tin 
lined lead tubes have been employed. At the beginning of 1884, 
Messrs. HAMBURGER of Utrecht, who made the tubes, stated that a 
number of blocks of Banca tin in their store had become grey and 
