L. Semann on Geological Phenomena in the Solar System. 41 
and of Reaumur’s porcelain, are striking examples of the ten- 
dency of molecules to group themselves in crystals even in the 
midst of solid masses, and we can thus readily understand the 
absence of vitreous substances among the older crystalline rocks. 
The great difficulty is to determine with exactness the propor- 
tion of the vacant spaces resulting from this change, since these 
will vary for each body, and probably also with the volume of 
the mass. Sulphur fused in an open vessel crystallizes slowly, 
the level of the liquid sinks a little, and after complete solidifi- 
cation the surface is covered with hollows resulting from the 
shrinking, whereas if cooled in a spherical shape these cavities 
would naturally be formed at the centre. Water and bismut 
as is well known, behave in a very different and remarkable 
manner, the first dilating eight or ten hundredths at the mo- 
ment of congelation, and the second one fifty-third. The only con- 
clusion to be drawn from these facts is that each body in solidifi- 
cation behaves in a different manner, and that for the solution of 
the question before us, we can only take into account the well 
own porosity of rocks. The problem, however, appears to me 
one of great importance in connection with theoretical geology ; 
if we admit with Deville that at the moment of crystallization, the 
density of rocks is in all cases augmented, we are forced to con- 
clude that all the crystalline masses formed at the surface of the 
liquid globe must have sunk and accumulated at the centre. 
sulphur, We should then have in place of a liquid globe sur- 
ified to the centre, a con- 
r haramerii 
: from a contraction similar to that which takes place in 
igneous rocks. The surface becoming solid while the interior is 
yet liquid, the natural contraction of this portion is prevented, 
and from this necessarily res 
vacant spaces thus produced, we find for iron a contraction of 
0075, for nickel 0-045 - for aluminum 0°041; for copes singel 
it results that an ingot of gold, the most solid obtained by the 
Am. Jour. Scr.—Szconp Serres, VoL, XXXIII, No. 97.—Jan., 1962, 
6 
necessary t 
absorb the whole atmosphere, would be only 0°004. From this 
