t of lead in 100000 parts of water; and this
acute Philosopher considers it as the most unequivocal test of lead that
we possess. Dr. Thomson remarks that "no other precipitate can well be
confounded with it, except sulphate of barytes; and there is no
probability of the presence of barytes existing in common water."

Carbonate of potash, or carbonate of soda, may also be used as agents to
detect the presence of lead. By means of these salts Dr. Thomson was
enabled to detect the presence of a smaller quantity of lead in
distilled water, than by the action of sulphuretted hydrogen. But the
reader must here be told, that the use of these tests cannot be
entrusted to an unskilful hand; because the alkaline carbonates throw
down also lime and magnesia, two substances which are frequently found
in common water; the former tests, namely, water impregnated with
sulphuretted hydrogen gas, and nascent sulphuretted hydrogen, are
therefore preferable.

It is absolutely essential that the water impregnated with sulphuretted
hydrogen, when employed as a test for detecting very minute quantities
of lead, be fresh prepared; and if sulphate of potash, or sulphate of
soda, be used as tests, they should be perfectly pure. Sulphate of
potash is preferable to sulphate of soda. It is likewise advisable to
act with these tests upon water concentrated by boiling. The water to
which the test has been added does sometimes appear not to undergo any
change, at first; it is therefore necessary to suffer the mixture to
stand for a few hours; after which time the action of the test will be
more evident. Mr. Silvester[26] has proposed gallic acid as a delicate
test for detecting lead.

FOOTNOTES:

[11] Dalton, Manchester Memoirs, vol. iv. p. 55.

[12] Marsden's History of Sumatra.

[13] Manchester Memoirs vol. x. 1819.

[14] Observations on the Water with which Tunbridge Wells is chiefly
supplied for Domestic Purposes, by Dr. Thomson; forming an Appendix to
an Analysis of the Mineral Waters of Tunbridge Wells, by Dr. Scudamore.

[15] It is absolutely essential that the tests should be pure.

[16] Philosophical Magazine, vol. xv. p. 252.

[17] Transactions of the Royal Society of Edinburgh, vol. viii. p. 259.

[18] Sir G. Baker, Med. Trans. vol. i. p. 280.

[19] Lamb on Spring Water.

[20] Medical Trans. vol. i. p. 420.

[21] Van Swieten ad Boerhaave, Aphorisms, 1060. Comment.

[22] Medical Comment. Dec. 2, 1794.

[23] Lambe on S