698 Chronicles of Science. [Oct., 
of attention when they bear upon a subject so shrouded in darkness 
as the nature of the undecomposed bodies. 
The passage of hydrogen gas through homogeneous bodies at a 
high temperature has lately attracted considerable attention on the 
Continent. Some time ago M. L. Cailletet communicated a note to 
the French Academy, in which he showed that hydrogen gas would 
pass through plates of wrought-iron several millimetres thick at a red 
heat. The author now shows that when the iron is cold, or at a 
temperature of only 210°C., hydrogen will not traverse a plate of only 
z;th millimetre in thickness. M. H. Ste. Claire Deville has lke- 
wise been continuing his experiments on the same subject. His 
method of observing the phenomenon is to fill a wrought-iron tube 
with nitrogen, and place it in a porcelain tube, through which a current 
of hydrogen is passed. Upon heating the whole in a furnace, and 
observing the pressure on the inside and on the outside of the iron 
tube, it is found that that of the interior may become almost double 
that of the exterior, in consequence of the hydrogen permeating the 
walls of iron and adding its pressure to that of the nitrogen. Unless 
the temperature is very high, no nitrogen passes out; but at very 
exalted temperatures, the author remarked that the internal and exter- 
nal pressures became equalized, in consequence of the intra-molecular 
spaces becoming so much dilated as to allow the nitrogen to pass 
freely. M. Deville conceives that if we knew the law of the dilatation 
of these inter-molecular spaces, we might determine the relative sizes 
of the molecules of hydrogen and nitrogen. 
The same indefatigable experimentalist has been also engaged 
with M. Troost in perfecting the means of determining high tem- 
peratures, and they describe a porcelain apparatus by which they 
measure a temperature reaching as high as 1530° C. The description 
is too long to be given here, but the account leaves no doubt that we 
have now a pyrometer capable of giving very exact indications, and 
likely to receive important applications. The authors record that, at 
the temperature above given, copper and silver seemed to be vaporized, 
and feldspar was fused to a perfectly clear glass. An iron nail, how- 
ever, showed no signs of fusion. 
It has often been observed that a diminution of density occurs 
when certain minerals are exposed to heat, and, in particular, it was 
announced by Magnus some years ago, that specimens of idocrase 
after fusion had diminished considerably in density without under- 
going any change of composition. Dr. Phipson has recently repeated 
this experiment of Magnus, and finds that it is true both for the whole 
family of garnets as well as for the minerals of the idocrase group. 
He finds that it is not necessary to melt the minerals, the change of 
density occurs upon their merely being heated to redness without fusion ; 
and Dr. Phipson has obtained the curious result, that the diminished 
density thus produced by the action of a red heat is not permanent, 
but that the specimens in the course of a month or less resume their 
original specific gravities. Thus three specimens of lime-garnet 
mellitite from Vesuvius, having an original density respectively of 
3°345, 3°350, and 3:°349, after being heated to redness for a quarter 
