| 
) 
. 
: 
4 
y 
C.—GEOLOGY. 67 
Temperature was measured by a platinum-platinum-rhodium electric 
pyrometer of which the sensitive part was immersed in the CaCO, which 
was being experimented on. Carbonic acid gas was provided in an 
ordinary steel cylinder such as is used for trade purposes; these can 
easily stand higher pressures (at ordinary temperatures) than those it 
was necessary to employ. ‘The gas in these cylinders is at fifty 
atmospheres pressure, but by immersing the cylinder in hot water the 
pressure could be raised sufficiently for the purposes of the experiment. 
_ All pressures were measured by an ordinary Bourdon gauge, such as is 
used for many purposes in the arts. Through the walls of the vessel 
the insulated wires of the electric furnace and pyrometer and the tube 
carrying carbonic acid gas were led by gas-tight junctions. The whole 
apparatus worked perfectly smoothly. It is a type of experimental 
plant which is already employed in many researches into the behaviour 
of substances at high temperatures under considerable gas pressures, 
and seems likely to play a large part in the progress of experimental 
geology in the near future. By slow stages it has reached its present 
development, and when we remember how many advantages we enjoy 
in experimental work to-day as compared with Sir James Hall, who 
was a real pioneer and had to invent all his apparatus and solve every 
difficulty for himself, we can appreciate more thoroughly the masterly 
ingenuity he displayed. 
As the outstanding uncertainty about Hall’s experiments is the 
question whether his carbonate of lime was actually melted or not, we 
_may pause to consider what evidence is accepted as sufficient on this 
point at the present day. In ordinary cases the proof of fusion would 
be that the mass became liquid, but as the charge is contained in a 
furnace inside a closed steel vessel it is impossible to examine it till the 
apparatus cools down and is opened up. In many cases also it is 
possible to rapidly cool the melt, by dropping it into water or mercury, 
and if it solidifies as a pure glass the proof of fusion is complete. The 
‘carbonate of lime, however, could not be chilled either directly or 
‘indirectly, and, furthermore, it seems clear that this substance crystal- 
lises so readily that to obtain solidification as a vitreous mass might be 
quite impossible. Reliance accordingly must be placed on a third ex- 
perimental method, that of reading the heating and cooling curves as 
recorded by the pyrometer. Change of state involves either the libera- 
tion or absorption of heat, and these may be ascertained without any 
difficulty. Fortunately the behaviour of carbonate of lime in this respect 
is quite satisfactory; it melts sharply at a definite temperature and 
erystallises very readily on cooling, so that the exact fusion point is not 
difficult to observe. Moreover, the microscopic appearance of the 
erystaliine masses produced is entirely in accordance with the belief 
that complete fusion had taken place. 
We may now consider what light modern research has thrown on the 
vexed question whether Hall succeeded in melting carbonate of lime, 
and on the value and accuracy of his experimental work generally. 
It is clear that Hall in his best experiments was able to prevent escape 
of gas from his gun-barrels. The fact that there was no significant 
loss of weight in the materials he used seems to prove this satisfactorily. 
