Page:Proceedings of the Royal Society of London Vol 1.djvu/376

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Au opinion having for some time prevailed, both in this and in other countries, to the prejudice of British salt as a preserver of animal food, and large sums of money being annually paid to foreign nations, in conformity to this opinion, for the supply of an article which Great Britain possesses beyond almost any other country in Europe, Dr. Henry has been induced to undertake the present inquiry, for the purpose of ascertaining whether the preference of foreign salt he founded on accurate experiments, or merely a matter of prejudice; and in the former case, whether any chemical difference could be discovered that would explain that superiority.

The subject is divided into three parts : in the ﬁrst of which are contained, general observations on the different kinds of salt manufactured, and the modes of preparing them. The second division contains the general statement of the results of the experiments, and conclusions deducible from them. But since a long detail of analytical processes must be uninteresting to very many persons, to whom the results will be acceptable, the description of the method employed by Dr. Henry, in his analysis, is reserved for the last section, in which (as the author observes) are not to be expected any novelties in science, but merely a. careful selection and employment of known processes.

The principal sources of the salt manufactured in this country are: the rock salt of Cheshire; the brine springs. which are also mostly in the same district; and sea water, which cannot be evaporated artiﬁcially with proﬁt, excepting where fuel is cheap.

Concerning the preparation of salt in Cheshire, Dr. Henry extracts a brief statement from an excellent history given by Mr. Henry Holland, in the agricultural report of the county of Chester; and he notices four varieties.

First. That which is called stoned or lump salt. Second. Called common salt. Third. Large—grained ﬂaky salt. Fourth. Fishery salt;-—the difference among these depending principally on the degree of heat used for evaporation of the brine.

The stoved salt is prepared by rapid evaporation at a boiling heat of 226° Fahrenheit, and it obtains its name from being subsequently dried in stoves after being Well drained.

For making common salt, the brine is evaporated at a temperature between 160° and 170°; and as the salt is consequently formed in somewhat larger grains, it is merely drained, and does not require to be dried in stoves as the former. The name of the third, or large-grained ﬂaky salt, implies a slower process of crystallization, and it is conducted at a. temperature of 130 or 140 degrees.

The last, or ﬁshery salt, which is the largest, is prepared at a heat