Under the above title almost the entire history of chemistry might
be easily comprehended. The gradual growth of definite meanings
attached to these terms has been coincident with the attainment of
accurate notions concerning the composition of bodies. To the ancient
philosophers sour wine, acetum vinæ, or acetum as it is still called,
was the only acid definitely known. When the alchemists became busy
trying to extract the virtue out of all substances they produced
several acids by distillation. These they called, for example,
spirit of vitriol, spirit of nitre, spirit of salt, meaning our
sulphuric, nitric, and hydrochloric acids respectively. They regarded
everything obtained by distillation as a spirit. When the theorists
came forward, Becher, Stahl, and their followers, they treated these
acids as original constituents of the substances from which they were
obtained. Thus, when sulphur was burned phlogiston was set free, and
acid remained. Lavoisier believed that the acidifying principle had
been discovered in oxygen, and it was on this theory that he gave that
element its name. But this idea broke down when Davy proved that there
was no oxygen in the so-called muriatic, or oxy-muriatic acid. It was
the subsequent recognition of the law of substitution which made it
clear that the acids are, in fact, salts of hydrogen or of some metal
substituted for the hydrogen.
The history of alkalies is as varied as is that of acids. The
distinction between caustic alkalies and mild alkalies was a problem as
far back as Dioscorides. By burning limestone caustic lime is produced.
It was not an unreasonable presumption that the fire had created this
causticity, and this theory was held with regard to all the alkalies
until it was proved by Joseph Black, in 1756, that the caustic alkali
was the result of a gas, fixed air, he named it, being driven off from
the mild alkali.
The ancient Jews prepared what they called Borith (translated “soap”
in Jeremiah, ii, 22, and Malachi, iii, 2) by filtering water through
vegetable ashes. Borith was therefore an impure carbonate of potash.
It is probable that the salt-wort was generally employed for this
purpose, and some of the old versions of the Old Testament give the
herb “Borith” as the proper sense of the passages referred to above.
In any case the alkaline solution produced from vegetable ashes was
used for bleaching and cleansing purposes. The Roman “lixivium” was
similarly prepared, and the process is still followed in some countries
where there are dense forests. The Arabic word “al-kali” was apparently
applied to the product from the word “qaly,” which meant “to roast.”
The earliest known use of the term is, however, found in the works
of Albertus Magnus, early in the thirteenth century. A process of
making caustic potash by filtering water through vegetable ashes with
quicklime is described in the works attributed to Geber, but this
is in a treatise now known to have been written in the thirteenth
or fourteenth century. It was only in 1736 that the three alkalies,
soda, potash, and ammonia, were definitely distinguished by Duhamel as
mineral, vegetable, and animal or volatile alkalies.
A formula for a solution of caustic potash was given in the P.L.,
1746, under the title of Lixivium Saponarium. Equal parts of Russian
potashes and quicklime were mixed, wetted until the lime was slaked,
water afterwards added freely, and after agitation the solution poured
off. This was ten years before Black’s classic investigation already
referred to. Before Black, and for some time afterwards, there were
several theories in explanation of the action of the lime on the
potashes. The lime had been tamed, but the potash had become more
virulent. One popular suggestion was that the lime had withdrawn a
kind of mucilage from the potashes; another that it had the effect
of developing the power of the potashes by a mechanical process of
comminution. A German chemist named Meyer, who vigorously opposed
Black’s conclusions, maintained that the lime contained a certain
Acidum Causticum or Acidum Pingue, which potashes extracted from it.
In the P.L., 1788, the process was altered by increasing the proportion
of the lime, and the product was described as Aqua Kali Puri.
Subsequently the proportion of the lime employed was reduced.
The word “salt” is traced back to the Greek “hals,” the sea, from which
was formed the adjective “salos,” fluctuating (like the waves), and
subsequently the Latin “sal.” Marine salt was therefore the original
salt, and salts in chemistry were substances more or less resembling
sea-salt. Generally, the term was limited to solids which had a taste
and were soluble in water, but the notion was developed that salt was
a constituent of everything, and this salt was extracted, and was
liable to get a new name each time. Salt of wormwood, for instance, is
one of the names which has survived as a synonym for salt of tartar,
or carbonate of potash. Paracelsus insisted that all the metals were
composed of salt, sulphur, and mercury, but these substances were
idealised in his jargon and corresponded with the body, soul, and
spirit, respectively.
Lavoisier was the first chemist who sought to define salts
scientifically. He regarded them as a combination of an acid with a
basic oxide. But when the true nature of chlorine was discovered it was
found that this definition would exclude salt itself. This led to the
adoption of the terms “haloid” and “amphide” salts, the former being
compounds of two elements (now the combination of chlorine, bromine,
iodine, cyanogen, or fluorine with a metal), and the latter being
compounds of two oxides. The names were invented by Berzelius. Since
then salts have been the subjects of various modern theories, electric
and other, but they are always substances in which hydrogen or a metal
substituted for it is combined with a radical. In a wide sense the
acids are also salts.