s, before it will be so much injured. It may be
worth while, to make farther experiments with respect to this property
of water.

In consequence of using the rain-water, and the well-water above
mentioned, I was very near concluding, contrary to what I have asserted
in this treatise, that common air suffers a decomposition by great
rarefaction. For when I had collected a considerable quantity of air,
which had been rarefied about four hundred times, by an excellent pump
made for me by Mr. Smeaton, I always found, that if I filled my
receivers with the water above mentioned, though I did it so gradually
as to occasion as little agitation as possible, a candle would not burn
in the air that remained in them. But when I used distilled water, or
fresh spring-water, I undeceived myself.

I think myself honoured by the attention, which, from the first, you
have given to my experiments, and am, with the greatest respect,

Dear Sir,

Your most obliged

Humble Servant,

London, 7 Dec. 1773.

J. PRIESTLEY.

POSTSCRIPT.

I cannot help expressing my surprize, that so clear and intelligible an
account, of Mr. SMEATON'S air-pump, should have been before the public
so long, as ever since the publication of the forty-seventh volume of
the Philosophical Transactions, printed in 1752, and yet that none of
our philosophical instrument-makers should use the construction. The
superiority of this pump, to any that are made upon the common plan, is,
indeed, prodigious. Few of them will rarefy more than 100 times, and, in
a general way, not more than 60 or 70 times; whereas this instrument
must be in a poor state indeed, if it does not rarefy 200 or 300 times;
and when it is in good order, it will go as far as 1000 times, and
sometimes even much farther than that; besides, this instrument is
worked with much more ease, than a common air-pump, and either exhausts
or condenses at pleasure. In short, to a person engaged in philosophical
pursuits, this instrument is an invaluable acquisition. I shall have
occasion to recite some experiments, which I could not have made, and
which, indeed, I should hardly have dared to attempt, if I had not been
possessed of such an air-pump as this. It is much to be wished, that
some person of spirit in the trade would attempt the construction of an
instrument, which would do great credit to himself, as well as be of
eminent service to philosophy.

FOOTNOTES: