Such are the various methods which have been re commended for ascertaining the nature and quantity of the different compounds contained in mineral wa ters. It is doubtful, however, if, by these means, we arrive at the exact quantity of the different substances, and it is still more doubtful, if we really procure them in the state of combination in which they exist in the mineral water.
It has been already remarked, that little reliance can be placed in the knowledge derived from imcompatible salts, as some salts which decompose each other, when their solutions are concentrated, exist together in solu tion, if a large quantity of the solvent be present.
This subject has been lately particularly investigated by Dr. Murray. From numerous experiments he has found, that salts, different from those known to be in solution, can be obtained by the evaporation of the sol vent. This has induced him to call in question the ac curacy of the modes of analysis explained, and to propose another method of determining the state of combination of the ingredients of mineral waters. See Transactions of the Royal Society of Edinburgh, vol. vii.
According to Dr. Murray, two views may be enter tained with respect to the state of combination of saline substances dissolved in water. When we have two acids and two bases contained in this fluid, these may be united by the pure force of the affinity, and those sub stances between which the most powerful attraction is exerted, will be combined. Thus, suppose rnuriatic and sulphuric acids, and soda and lime, be dissolved in the same fluid, if the pure force of the affinity operate, the compounds formed will be sulphate of lime and muriate of soda. We know, however, that affinity is much influenced by external circumstances ; when, therefore, any of these operate, compounds may be formed, different from those which the pure force of the affinity a ould generate. When the above mention ed substances are dissolved in a small quantity of water, the compounds stated are formed, but if the quantity of fluid be great, it is possible that it may, by its quantity, influence the affinity, and other compounds will be the result. When the quantity of fluid is small, or when we evaporate the fluid by which the cohesion is allow ed to operate, we find invariably that the compounds existing in the water are those, between the particles of which the strongest cohesion is exerted. The reverse,
it is probable, is the case, when the quantity of water is great, the. quantity of the fluid causing the formation of substances, between the particles of which there is little power of cohesion, so that the most soluble salts are formed. When the above-mentioucd substances, then, are dissolved in a large quantity of water, instead of sulphate of lime, and muriate of soda, we will have mu riate of lime and sulphate of soda. The same it is sup posed is the case, when carbonate of lime and muriate of soda are procured by evaporation, these salts having been formed by the decomposition of the carbonate of soda and muriate of lime, which were the salts that ex isted in the water, when a large quantity of the solvent was present. If carbonate of magnesia and muriate of soda be procured, the salts in solution were probably carbonate of soda and muriate of magnesia. In all mine ral waters, the quantity of fluid is large in proportion to the saline ingredients; these, therefore, are probably united, so as to form the most soluble salts.
This view of the state of combination of the substan ces in solution in a large quantity of fluid, is supported by several facts. Thus, by the evaporation of some mineral waters, a quantity of a substance sparingly solu ble, as of sulphate of lime, is obtained in greater quan tity than we know water can hold in solution. It must therefore have been formed during the evaporation by the operation of new affinities. It usually happens also, that when this is the case, the deposition of the sub stance commences at a certain stage of the evaporation, and that, after the fluid is farther evaporated, the reposi tion ceases. At this point, then, it would appear that the substance deposited begins to be formed.
Another strong argument in favour of the opinion of Dr. Murray is, that different products are obtained from a mineral water according to the mode of analysis. Sea-water, for instance, by slow evaporation, affords muriate of soda, muriate of magnesia, and sulphate of magnesia. When the fluid is evaporated to dryness, and the residue is subjected to the action of alcohol, the products are, muriate of soda, muriate of magnesia, and sulphate of soda. It is likely, then, that during the evaporation of the fluid new affinities may exert their influence, so that salts, different from those which exist in solution, may be obtained.