60 SECTIONAL ADDRESSES, 
famous apophthegm has often been quoted about those who ‘ judge 
of the great operations of the mineral kingdom by kindling a fire and 
looking in the bottom of a crucible.’ In deference to the feelings of 
his master and his father’s friend, Sir James Hall, with admirable 
self-restraint, decided not to undertake experimental investigations in 
opposition to Hutton’s expressed opinion. With a few months’ inter- 
ruption in 1800 they were continued till 1805. A preliminary account 
of the results was communicated to the Royal Society of Kdinburgh 
on August 30, 1804, andthe final papers submitted on June 3, 1805. 
Hall states that he made over 500 individual experiments and destroyed 
vast numbers of gun-barrels in this research. 
The method adopted was to use a muffle-furnace burning coal or 
coke and built of brick. No blast seems to have been employed. The 
chalk-powder was enclosed in a gun-barrel cut off near the touch-hole 
and welded into a firm mass of iron. The other end of the barrel could 
be kept cool by applying wet cloths, and as it was not in the furnace 
its temperature was always comparatively low. Various methods of 
plugging the barrel were adopted ; at first he used clay, sometimes with 
powdered flint. Subsequently a fusible metal which melted at a tem- 
perature below that of boiling water was almost always preferred. Borax 
glass with sand was used in some of the experiments, but it was lable 
to cracking when allowed to cool, and consequently was not always gas- 
tight. It was essential, of course, that in sealing up the gun-barrel, 
and in subsequently removing the plug, the temperatures should never 
be so high as to have any sensible effect on the powdered chalk or lime- 
stone. Hall tried vessels with screwed stoppers or lids at first, but 
never found them satisfactory. 
In the gun-barrel there was always a certain amount of air enclosed 
with the chalk. Very early in the experiments it was shown that if 
no air-space was provided the fusible metal burst the barrel. No means 
was found to measure the size of the air-space accurately, but approxi- 
mately it was equal to that of the powdered chalk used in the experi- 
ment. If the air-space was too large, or if there was an escape of gas, 
part of the chalk was converted into lime. 
As each experiment lasted several hours the temperature of the chalk 
was approximately equal to that of the part of the muffle in which it 
was placed. Pyrometry was as yet in its infancy. | Wedgwood had 
invented pyrometric cones and Hall had heard of them, but apparently 
at first he was not in possession of a set. He made his own cones, 
as nearly similar as possible to those of Wedgwood, and subsequently 
obtaining a set of Wedgwood’s cones he standardised his own by com- 
parison with them. His gun-barrels of Swedish and Russian iron (‘ Old 
Sable ’) were softened, but seldom gave way except when the internal 
pressures were of a high order. Some of the gun-barrels seem to have 
been used for many experiments without failure occurring. As Hall 
made his own pyrometric cones, and we have no details of their com- 
position and the method of preparation, it is not possible to do more than 
guess at the temperatures to which his powdered lime and chalk were 
exposed. There is no doubt that by constant practice and careful 
oo he was able to regulate the temperature within fairly wide 
imnits, 
