hi the Length of the Pendulum Vilratilig Seconds^ 325 
M. Biot’s experiments have not yet been published ; but it sif- 
fords me much gratification to learn, that the acceleration of the 
pendulum between London and Unst, computed by M. Biot, 
from his observations at Unst and those at Formentara, using 
5^15 for the compression, differs only 0®,6 from the result of 
my experiments ; a difference which may probably be referred 
to the superior, density of Unst, con^ared with that of the sub- 
strata of London.” 
Art. XX. — On the Discovery (f Potash in Sea Water. In a 
Letter from Dr Wollaston to Dr Marcet i 
^Fhen Dr Marcet was beginning the chemical part of his 
researches respecting the specific gravity and temperature of sea 
waters, Dr Wollaston suggested to him the probability that 
traces of potash might be found in them. In order to examine 
this point. Dr Marcet supplied him with various specimens of 
sea water ; and in the following note. Dr AVollaston communi- 
cated to him the results of his examination. 
The expectation which I expressed to you, that potash 
would be found in sea-water, as an ingredient brought down by 
rivers from the decay of land plants, is now fully confirmed by 
experiments on waters obtained from situations so remote from 
each other, as to establish its universality. 
, ‘‘ There is no difficulty in proving the presence of this ingre- 
dient by muriate of platina. For though the triple muriate of 
platina and potash is so soluble that this reagent causes no pre- 
cipitate from sea-water in its ordinary state, yet when the water 
has been reduced by evaporation to about Jth part, so that the 
common salt is beginning to separate by crystallization, the mu- 
riate of platina then causes a copious precipitate. 
‘‘ If this precipitate be mixed with a little sugar and heated, 
the platina is reduced, and muriate of potash may be separated 
from it by water, and the nature of its base shown by its yield- 
ing crystals of nitrate of potash with nitric acid. 
• From Dr Marcet’s paper “ On the Specific Gravity and Temperature of Sea- 
Waters,” in the FAzl. Trans. 1819, p. 200. 
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