Acids, Alkalies, and Salts.
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.