62 SECTIONAL ADDRESSES. 
was also some carbonic oxide. About this time he was using bars of 
Russian iron into which a long cylindrical cavity had been bored. He 
then tried other volatile ingredients such as nitrate of ammonia, car- 
bonate of ammonia, and gunpowder. In January 1804 he was able to 
convert chalk into firm limestone at a temperature about 960° (melting- 
point of silver) in presence of small quantities of water with a loss of 
less than one-thousandth part of the chalk used. 
Finally he attempted to measure the pressure which was necessary 
to effect re-crystallisation under the conditions of his experiments. No 
pressure gauges were available at that date, and after many trjals he 
employed a stopper faced with leather and forced against the mouth of 
his iron tube by means of weights acting either directly or through a 
lever. He ultimately succeeded in obtaining gas-tight junctions under 
pressures ranging from 52 up to 270 atmospheres, and concluded that 
52 atmospheres was the least pressure which could be satisfactory. 
This is equal to the pressure of a column of water 1,700 feet high or 
to a column of rock 700 feet high. A ‘complete marble’ was formed 
at a pressure of 86 atmospheres and carbonate of lime ‘ absolutely 
fused ’ under a pressure of 173 atmospheres. 
In reviewing these classic experiments after a lapse of 120 years 
we feel that there are many points on which we should have liked more 
detailed information. One essential, for example, is exact chemical 
analysis of all the materials employed. Even chalk is variable in com- 
position to a by no means negligible extent. Oyster shells and peri- 
winkle shells contain organic matter, which would account for the 
considerable loss in weight they always exhibited. The use of glass 
tubes was a defect in the early experiments, afterwards remedied by 
employing platinum vessels. Although in all the experiments the 
charge was weighed it seems clear that at first at any rate the materials 
were not carefully dried. In the experiments with water it was seldom 
possible to provide absolutely against the escape of moisture when the 
fusible metal was introduced. Most of all we may regret the inadequate 
means of measuring the temperatures at which the experiments were 
conducted. The measurements of pressure were made by the simplest 
possible means, and it was only by great experimental skill and care 
that even approximate results could be obtained. 
Such criticisms, however, do not mar the magnificent success of 
Hall’s experiments. For nearly a hundred years, in spite of the advance 
of physical and chemical science, no substantial improvement on his 
results was attained. His work was immediately recognised as trust- 
worthy and conclusive, and became a classic in the literature of experi- 
mental geology. Although not exactly the founder of this school of 
research, for Spallanzani and De Saussure had made fusion experiments 
on rocks before his time, he placed the subject in a prominent position 
among the departments of geological investigation, and did great service 
in supporting Hutton’s theories by evidence of a new and unexpected 
character. 
As Hall himself has told us, there were critics who before the com- 
plete account of his researches in carbonate of lime was published had 
challenged the accuracy of some of his conclusions. The ground seems 
