Note 390 — PHARMACOLOGIA. (continued 2)

The law which determines such combinations has been investigated with singular ingenuity and success by Dr. Murray, (Transactions of the Royal Society of Edinburgh, 1816). Berthollet had already established the important fact, that combinations are often determined by the force of cohesion, in such a manner, that in principles acting on each other, those on which this force operates most powerfully, in relation to the fluid which is the medium of action, are combined together; hence from a knowledge of the solubility of the compounds which substances form, we may predict what combinations will be established when they act on each other, those always combining which form the least soluble compounds. It is for the extension of these views, and for the useful application of them, that we are indebted to Dr. Murray, who justly observes that if the force of cohesion can so far modify chemical attraction, as to establish among compound salts dissolved in any medium, those combinations whence the least soluble compound are formed, we are entitled to conclude that the reverse of this force, i. e. the power of a solvent, may produce the opposite effects, or cause the very reverse of these combinations to be established, so that in a concentrated medium the least soluble will be formed, and in a dilute one, the more soluble compounds will be established. Hence follows the simple rule by which the actual state in which saline bodies exist in a solution may be determined, viz. that in any fluid containing the elements of compound salts, the binary compounds existing in it will be generally those which are most soluble in that fluid, and the reverse combinations will only be established by its concentration favouring the influence of cohesion. It appears that by simply evaporating a saline solution we may produce changes in its composition, and obtain products which never existed in its original state of dilution; thus, suppose muriate of magnesia and sulphate of soda to be dissolved in water, as is actually the case in the water of the ocean, and the solution to be concentrated by evaporation from heat; the combinations of sulphate of magnesia and muriate of soda, being on the whole less soluble in water, this circumstance of inferior solubility, or the force of cohesion thus operating, actually determines the formation of these compounds; and the production of sulphate of magnesia from the bittern is to be explained upon this principle. Since it appears therefore that the influence of solubility is most important, temperature, to whose dominion it is under all circumstances subject, must necessarily be alike powerful; let us exemplify this fact by the action of the very salts under consideration; it has been just stated that muriate of magnesia and sulphate of soda decompose each other in a concentrated solution at a high temperature, producing muriate of soda and sulphate of magnesia, but at temperatures below 32° the reverse actually takes place, muriate of soda and sulphate of magnesia reacting, and being converted into sulphate of soda and muriate of magnesia; a fact evidently owing to the relation of the solubility of these salts to temperature. Muriate of soda has its solubility scarcely altered, either by heat or cold; sulphate of soda is, in these respects, completely the reverse; hence at an elevated temperature, muriate of soda is the least, and sulphate of soda the most soluble salt, whilst at a low temperature, the reverse of this happens. All the circumstances of this investigation are most interesting; the medical practitioner will at once perceive its importance, as enabling him to appreciate the real nature of saline solutions, and even in many instances to preserve their identity. See Aquæ Minerales.