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 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