Nitrum, _Nitre_ (or _Soda_?), is possessed of intermediate powers between the aphronitrum and salts. When burnt it becomes more attenuate and discutient. If taken internally it cuts and attenuates the thick and viscid humours more than any other. The Chalestræan (so called from Chalestra, a place in Thessalonica) is more acrid and desiccative, so that it is used for defluxions from the head, and for arthritic complaints. * * * * * COMMENTARY. The following extract from Pliny’s description of the nitrum clearly indicates the substance to which he applies it: “Non est differenda et nitri natura, non multum a sale distans. Exiguum fit apud Medos, canescentibus siccitate convallibus, quod vocant halmyrhaga. Optimum copiosumque in Litis Macedoniæ quod vocant Chalestricum, candidum, purumque, proximum sali. Lacus est nitrosus, exsiliente e medio dulci fonticulo. In Ægypto autem conficitur multo abundantius, sed deterius. Fit pæne eodem modo quo sol, nisi quod salinis mare infundunt, Nilum autem nitrariis.” (H. N. xxxi, 46.) Compare the above with the following account of the native carbonate of soda: “In Egypt it is plentifully found in what are called _the Lakes of Natron_. These lakes, six in number, are westward of the Nile, not far from Terrana, in a valley surrounded by limestone. The carbonate and muriate of soda exist together in these waters; but when the water is diminished by natural evaporation, these salts are deposited in distinct layers.” (Brogniart, in Cleaveland’s Mineral., 133.) It would be superfluous to bring any further proof that the ancient nitrum generally applies to our natron or the native carbonate of soda. Were the Greeks, Romans, and Arabians then entirely ignorant of saltpetre or the nitrate of potash? This seems incredible, considering how largely it is distributed in Europe, Asia, and Africa; and, therefore, we cannot bring ourselves to think that this mineral can have entirely escaped the observation of the ancients; and if this were the proper place for such a disquisition, we believe that we could state satisfactory reasons for drawing the conclusion, that “nitrum” was applied to it as well as to the other mineral. But at the same time, as mentioned above, we are quite satisfied that by _litrum_ and _nitrum_ the medical authors generally, if not universally, meant the native carbonate of soda. Dioscorides, with regard to its medicinal powers, merely states of it that it has alterative or metasyncritic powers. (v, 129.) We have given his views respecting the _Aphronitrum_ under that head. Our author abridges Galen’s account of it, but omits to mention, as stated by Galen, and also by Aëtius, that the different kinds of nitre were popularly used as counter-agents in cases of poisoning by mushrooms. All the Arabians treat largely of this substance generally under the name of _baurach_. See Avicenna (ii, 2, 84); Rhases (Cont. l. ult, iii, 115); Mesue (De Simpl. xvii); Serapion (c.