C.—GEOLOGY. 63 
~ to have been that the materials he worked with were impure, and that 
the glass or porcelain vessels in which the powdered chalk was placed 
were visibly acted on during the experiment. Hall recognised the 
_ justice of these conclusions in so far that he made further experiments 
aS = 
on the purest precipitated carbonate of lime that he could obtain, and 
he used platinum vessels instead of glass. These changes admittedly 
made success more difficult to attain, but he considered that he ulti- 
mately was able to fuse the pure chemical in platinum vessels with only 
a negligible loss of weight by escape of carbonic acid. This seems to 
have silenced criticism, and with the gradual acceptance of most of 
Hutton’s theories the controversy died down for a time. 
Many attempts were made to repeat Hall’s experiments during the 
next eighty years with varying degrees of success. No one was able to 
secure perfectly gas-tight stoppage of porcelain or iron tubes as Hall 
did, though they had the record of his experiments to help them, a fact 
which shows how extremely skilful Hall was in experimental practice. 
But various authors found that chalk, powdered limestone, and even 
pure Iceland spar powder or precipitated carbonate of lime could be 
converted into a firm coherent mass by heating in an open furnace. It 
was also claimed that lithographic limestone became a crystalline rock 
resembling marble when heated before a blowpipe under certain con- 
ditions. Whether the mass was actually fused was not expressly proved 
_ by any of these experiments, and in time it came to be recognised that to 
make a limestone from powdered chalk it was not necessary that melt- 
ing should take place. On the other hand, it was contended that when 
- ‘Hall’s experiments had resulted in the production of a vesicular or 
frothy mass which showed evidence that it had dripped or flowed, or 
that it had been spattered in drops about his apparatus, as he concluded 
from the appearance presented in certain of his experiments, there was 
some reason to believe that chemical action had taken place between 
the silicates of his tubes of glass or porcelain, or the pipeclay stems 
in which the drops of water were contained, and the carbonate 
of lime, with the formation of readily fusible compounds. Hall, of 
course, was perfectly aware that his carbonate of lime combined with the 
ingredients of glass, porcelain, pipeclay, refractory cones, and silica. 
He had noticed that in many experiments. What was necessary was a 
complete quantitative chemical analysis of some of his fused masses, to 
_ show that they were carbonate of lime and nothing else. This he never 
performed. He had his specimens of artificial marble cut and polished, 
thus testing their hardness, their crystalline structure, and their trans- 
parency. He noted also how far the specimens were permanent in dry 
air, and found that very frequently they disintegrated owing to the 
presence of a considerable proportion of caustic lime. In many cases 
also he threw part of the mass into acid and observed complete solution 
‘with effervescence of carbonic acid gas. He trusted apparently to deter- 
mining whether there had been loss of weight, by escape of carbonic acid 
gas either during the experiment or subsequently on opening the gun- 
barrel, and argued that if the tube and its contents had the same weight 
after and before the experiment there could have been no chemical 
