What the _alumen rotundum_ was, is not easily conjectured. Dioscorides says, that it was sometimes made artificially; but that the artificial alumen rotundum was not much valued. The best, he says, was full of air-bubbles, nearly white, and of a very astringent taste. It had a slaty appearance, and was found in Egypt or the Island of Melos. The _liquid alumen_ was limpid, milky, of an equal colour, free from hard concretions, and having a fiery shade of colour.[98] In its nature, it was similar to the alumen candidum; it must therefore have consisted chiefly, at least, of sulphate of alumina. [98] See Dioscorides, lib. v. c. 123. Plinii Hist. Nat. xxxv. 18. Bitumen and naphtha were known to the ancients, and used by them to give light instead of oil; they were employed also as external applications in cases of disease, and were considered as having the same virtues as sulphur. It is said, that the word translated _salt_ in the New Testament--“Ye are the salt of the earth: but if the salt have lost his savour, wherewith shall it be salted? It is henceforth good for nothing, but to be cast out, and to be trodden under foot of men”[99]--it is said, that the word salt in this passage refers to asphalt, or bitumen, which was used by the Jews in their sacrifices, and called _salt_ by them. But I have not been able to find satisfactory evidence of the truth of this opinion. It is obvious from the context, that the word translated _salt_ could not have had that meaning among the Jews; because salt never can be supposed to lose its savour. Bitumen, while liquid, has a strong taste and smell, which it loses gradually by exposure to the air, as it approaches more and more to a solid form. [99] Matthew v. 13.--“Ὑμεις εστε το ἁλας της γης· εαν δε το ἁλας μωρανθη, εν τινι ἁλισθησεται· εις ουδεν ισχωει ετι ει μη βληθηναι εξω, και καταπατεισθαι ὑπο των ανθρωπων.”