SCIENTIFIC SUMMARY. 
21 
paper is intended to be preliminary, the general conditions and difficulties 
of the experiments, and some of the results obtained, being only recorded. 
The galvanic resistance of the metallic sulphides examined, whether in the 
crystalline form or otherwise, was found to vary with the direction, strength, 
and duration of the current which passed through them. 
Electrostatic Induction. — The 11 Comptes Rendus ” (Ixxix. p. 1071) contains 
a paper of interest on the above subject. M. Neyreneuf has repeated with 
a Holtz the experiments of MM. Yerdet and Masson, to determine the 
direction of the induced current with the electric egg. Two Matteucci discs 
contain, one the inducing, the other the induced spiral. This latter is put 
in connection with the two extremities of a cylindrical Greissler tube 
•50 cms. long. The first spiral is connected by one end with the negative 
armature of the machine, while the other is attached to one plate of a con- 
denser with air film, whose collector communicated with the positive arma- 
ture. The spark leaps between the two armatures of the machine from 
which the condensers have been removed. Two currents in opposite 
directions traverse the inducing spiral under these conditions, the one to 
charge, the other to discharge the condenser. Those produced in the second 
spiral illuminate the Geissler tube very brilliantly. With an explosive dis- 
tance of 5 cms. we observe clearly the difference in appearance of the two 
poles ; but we *can, by diminishing it continuously, produce three separate 
inversions at the instants when the appearance of the two poles is identical. 
When the striking distance is 3 cms. large and widely separated strati- 
fications are formed, but presenting before the last inversion all the charac- 
ters produced by a Ruhmkorff coil. The same effects are produced by 
altering the distance of the spirals or of the plates of the condenser. These 
condensers show that the phenomenon in question bears no relation to the 
lateral discharge, characterised especially by the constancy of the direction 
of the current it produces. 
Does the Thermal Conductivity of Mercury vary with the Temperature ? — 
The following important note appears in the “ Academy ” (March). The 
author remarks that the paper it refers to, which appears in the “ Philoso- 
phical Magazine” for March, was originally communicated to “ Poggendorff’s 
Annalen.” According to Wiedemann and Franz, the metals have equal 
conducting power for heat and electricity, and since we know from numer- 
ous experiments that for electric conductivity a very marked variability 
with the temperatures takes place, it follows, if the statement of Wiedemann 
and Franz be true, that there will be found for the thermal conductivity of 
most metals a variability with temperature in about the same degree. On 
the other hand Lorenz has asserted the independence of temperature of the 
heat-conductivity of pure metals which remain homogeneous, and accounted 
for the observed variations by assuming the development of thermo-electric 
currents in consequence of unequal heating of the metals. To decide this 
question it was necessary to employ a pure metal which remains homoge- 
neous, and mercury was accordingly selected as being the only known metal 
satisfying the condition. Herwig’s experiments show that between 40° and 
160° C. the heat-conducting power of pure mercury is perfectly constant, 
and so far confirm the results of Lorenz. The author is occupied with the 
arrangement of experiments the object of which is to ascertain how far 
solid metals differ in their behaviour from mercury. 
