tion, temperature, evaporation, or quantity o f frefh water
running into i t ; beiides' this, I have made feveral experiments,
but never found fait at the mouth of rivers,
where they difembogue into the fea. It is true, that
fometimes after diftillation and evaporation I have found
a thoufandth part of common fait, and I once difcovered
as a refiduum, a little nitre ; but this proves nothing, and
with refpeft to the nitre, I coniider it to be a refiduum of
common fait, being perfuaded that this may change its
acid and alkaline bails, and become nitre, with motion
and ebullition,and reciprocally nitrous, and the alkaline
bails change into common falt(#).
(«} It would have feeen acceptable to the chemical readers, i f Mr. Bowles had favoured us
with any fufficient reafon to fupport this belief.
L E T T E R XX.
Olfervations made by Don Guillermo Bowles, on the roundne/s o f pebbles
in the beds o f rivers.
r I'^HE pebbles o f which I am now going to fpeak,
are thofe which are commonly found in moil
places without angles, or points, and though not per-
feftly round, have yet more or lefs that form, with a
fmooth furface. They are compofed of various matter,
and the firR idea which occurs o f their having acquired
this fmoothnefs, is from their rubbing againit each other,
or fome harder body ; this being the method we ufe in
order to give fuch a poliih to any fubRance, and as thefe
Rones are frequently found in large quantities in the beds
of great rivers, it is very eafy to fay they have been
brought down by the waters, and become fmooth by
their conflant frhfiion. For my part, I was always of
this opinion till my arrival at Aranjuez, where I difcovered
the fallacy of this reafoning, for I clearly perceived that
the Rones in .the bed o f the Tagus never moved from
their places, This of courfe daggered my former belief
; I was confirmed in my new principles by a variety!
o f obfervations, but to avoid being prolix I ihall only
D d d 2 offer