1866. | Phystes, 123 
of the compressed line splits the glass nearly through; but when 
the glass is thick and rigid, as plate-glass, unless the sheet is bent 
back and broken through immediately after the cut, greater diffi- 
culty will be experienced if allowed’ to remain for a time, for the 
compressed line of glass will speedily tear up the portion on both 
sides, leaving a wide ragged groove in place of the original clean 
and scarcely visible line. 
A paper on the Spectrum of Nitrogen has been communicated to 
the Parisian Chemical Society by M. Waltenhofen, who states that 
in an atmosphere of nitrogen properly rarefied, the violet rays dis- 
appear before the blue and green. The author's observations lead 
him to believe that nitrogen is a compound body. This opinion is 
gradually gaining ground amongst physicists who have paid at- 
tention to the electrical spectra afforded by nitrogen under varying 
conditions of rarefaction and intensity. 
Heat.—A discussion has lately taken place before the Man- 
chester Literary and Philosophical Society respecting the possi- 
bility of utilizing the internal heat of the earth as a source of motive 
power. Mr. G. Greaves, M.R.CS., at the October meeting of the 
Society, stated that it had been very generally admitted that coal 
will not cease to be furnished because of the exhaustion of the stores 
of the mineral now existing in the coal measures; and further, that 
the obstacles to the continued working of the mines will not be 
engineering difficulties. The increased depth from which the coal 
will have to be brought may add to the cost, but at that increased 
cost it will still be for a long time obtainable. The author con- 
sidered the real unsurmountable obstacle to be the high tem- 
perature of the lower portions of the carboniferous strata. The 
temperature had been shown to be at a depth of 4,000 feet, at least 
120° Fahr., a degree of heat im which human beings cannot exist 
for any length of time, much less use any exertion. It had oc- 
curred to the author to inquire whether the very agency which will 
prevent the continued supply of fossil fuel might not be made the 
means of rendering that supply unnecessary—whether, in short, the 
internal heat of the earth might not to some extent be utilized. One 
or two modes of doing this had presented themselves to his mind. 
One of these might, he conceived, be the direct production of steam- 
power by bringing a supply of water from the surface m contact 
with the heated strata by means of artesian borings or otherwise. 
This elicited a letter from Sr J. F. W. Herschel, saying that 
by employing condensed air, conveyed through conducting pipes, as 
a mode of working machinery at that depth—provided the air 
immediately on its condensation, and before its introduction imto 
the pit, were drained of the heat developed in the act of condensa- 
tion, by leading it, in pipes exposing a large external surface, 
