68 SECTIONAL ADDRESSES. 
His latest experiments with arrangements for measuring the pressure 
were conducted in a manner far less likely to obtain complete retention 
of the gas than his early experiments, but they show that he had 
obtained a useful first approximation to the pressures involved, and 
that somewhere between 100 and 150 atmospheres was the dissociation 
pressure of CaCO, on melting. So crude was his apparatus, to modern 
ideas, that it is wonderful he obtained any results at all. 
‘The necessity of providing an air space to prevent bursting of the gun- 
barrels by expansion of the fusible metal makes it certain that lime was 
always present in his melt, and as the air space was never accurately 
measured it is not certain to what extent dissociation took place. He 
was working therefore with mixtures of CaO and CaCO,, and the tem- 
perature of the fusion at which he aimed was the eutectic point at 
1218°C. In most cases he probably had excess of lime in his melt, 
but in his best experiments he was either very near the eutectic mixture 
or on the carbonate of lime branch of the curve. The effect of the water 
which he introduced may have been to lower the melting-point slightly, 
though there are no very exact experimental results at the present time 
to indicate the magnitude of this effect ; probably it was not very great. 
If then he was able to reach a temperature of 1218° C. he may be said 
to have succeeded. Everything depends on. the conditions in his 
furnace, and this was determined by the design of the furnace, the nature 
of the fuel, and the draught. Apparently he did not use a blast, and 
we are not informed as to the chimney. His furnace and muffle seem 
to belong to a pattern which has been long employed for refining silver 
and gold and for assaying copper. Now copper melts at 1082°C., and 
it is open to doubt whether a muffle-furnace of this type will give a tem- 
perature of 1218°C. It seems just possible that the melting-point of 
carbonate of lime may have been actually reached under the best working 
conditions. The question will never be settled. Hall’s pyrometers were 
the least satisfactory part of his apparatus, and all his critics are agreed 
that it is impossible to interpret the results that they gave. Without 
an exact knowledge of the materials, structure, and dimensions of his 
furnace, his fuel and his draught, we cannot reproduce the conditions 
under which he was working, and his descriptions of his methods are 
too incomplete to settle the point. 
The determination of the actual melting-point and vapour pressure 
of CaCO, is a question, however, which interests the physical chemist 
more than the geologist, and there is little evidence to show that the 
eutectic mixture of lime and carbonate of lime has a distinct importance 
as a component of rocks. Though Sir James Hall may not have clearly 
realised it, he had established a truth of far higher value to geologists. 
He had shown that at comparatively low temperatures such as the 
melting-point of silver, which is 960°C., a fine grained aggregate of 
calcite will readily recrystallise. If the mineral is an incoherent powder 
it will agglutinate into a firm coherent mass. At slightly higher tem- 
peratures it becomes plastic, so as to assume the shape of the vessel 
which contains it, and loses any angularities or irregularities of its 
surface. These temperatures are about the same as those exhibited by 
ordinary lavas, and must be quite common in the vicinity of under- 
