334 Mr. Knox’s experiments and observations 
article, and to whom I sent two pieces, desiring to know, from 
him, which of the two specimens was the best ; answered, 
that both were excellent, but that the lighter was the best. 
He had no suspicion that they were an artificial product. If 
the analysis of pitch-stone and pumice be considered together, 
it will be found that the water, bitumen, and lime, in the 
former, are the substances which alter the per centage. 
How the lime was got rid of by the volcanic heat is not 
easily accounted for ; but it is conceivable that it may have 
been acted on by the muriatic acid fumes, which, we know, 
accompany volcanic fires. Besides, lime may yet be found in 
some pumice, and none of that earth in some pitch-stones. As 
yet, I believe, there have been but two pitch-stones, the Meissen 
and the Newry, analysed, and the former, only, by a chemist 
of authority. It must be acknowledged, however, that it is 
difficult to imagine, on this hypothesis, how the alkali re- 
mained un-muriated, while the lime united to the muriatic 
acid. 
It appears to be a condition, in converting a stone into 
pumice, that it should contain a volatile substance, which can 
only be removed by the same degree of heat which is at the 
same time necessary for producing that sort of semi-vitrifica- 
tion in the mass which renders it coherent, hard, and porous. 
If a stone contains only water, pumice is not formed, because 
that is driven off by a red heat, which does not act upon the 
earths of which the stone is composed. Drive off the water 
at a red heat, and you have a harder but incoherent mass : 
increase the heat, and you have, as in the case of alumine, 
a more compact and denser substance, or else, when the in- 
gredients of the stone favour vitrification, a glass. 
