We have thus, from very scanty records concerning salt, essayed to clear up, though very inconclusively, and I fear unsatisfactorily, certain points which have been unnoticed, by reason, I think, of the dense obtenebration with which the subject is surrounded; for it has hitherto baffled the researches of the geologist to discover its pristine source, and neither do we know who first used it as a condiment. The chemist can experimentalise with this inorganic substance to detect the presence of other bodies, and he knows its worth in the laboratory; but as for its origin, he is as much in the dark as the geologist. CHAPTER III. SALT AS A CHEMICAL, THERAPEUTICAL, AND TOXICOLOGICAL AGENT. As a chemical agent, and from the manufacturing uses to which it is now put, salt is a most invaluable article from a scientific as well as from a commercial point of view. I will therefore draw the attention of my reader to its chemical properties; I will then allude to a few drugs which are partially derived from salt or the chloride of sodium; and will cursorily notice one great staple of commerce which owes the rapidity of manufacture to its sole agency, including some remarks on it as a poison. _Chlorine_ gas, which is obtained from the _chloride of sodium_, was discovered by Scheele in 1777, who named it _dephlogisticated muriatic acid_. Berthollet in 1785 termed it _oxygenated muriatic acid_. Sir Humphry Davy called it _chlorine_ (from χλωρὸς, green) on account of its colour, and it has kept this name ever since. We thus see that salt is of great use to the chemist, for he not only obtains _chlorine_ gas from it, but also _hydrochloric acid_, a most useful and efficacious drug in the treatment of some hepatic diseases. _Chlorine_ also enters into combination with other chemical substances known as _chlorides_ and _chlorates_, _sub-chlorides_ and _per-chlorides_; for instance, we have the _chloride of ammonium_ and the _chlorate of potash_; we also have the _sub-chloride of mercury_, or _calomel_, and the _perchloride of mercury_, or _corrosive sublimate_, with various others. According to Pereira, _hydrochloric acid_ was known to Djafar, or Geber, an Arabian chemist who flourished in the eighth century, and whom Roger Bacon calls _magister magistrorum_. Everyone is acquainted with the _chloride of lime_, a substance so generally used for household and disinfecting purposes, that I need only mention it; besides this, there are other salts with which _chlorine_ enters into combination. Formerly, to bleach cotton it was required to expose the material to the action of the sun and air, rendering the process long and tedious, as it took on the average quite six or eight months, and likewise a large surface of land was necessary for the operation.