LECTURE XXIX. CHEMISTRY. SALTS. THE objects of chemistry are commonly divided into salts, earths, combustibles, metals, waters, and vegetable and animal substances. By taking them in this order, we shall find that those of a similar nature, and which have most qualities in common, are united together. Salts are commonly defined to be substances which are fusible, volatile, soluble in water, not inflammable, and sapid when applied to the tongue. These qualities are united to distinguish them from other bodies which will be afterwards mentioned. The first objection that has been made to this definition is, that their, fusibility or volatility are not distinguishing characters, as we have reason to conclude there is no species of matter incapable of both: but what is meant here is, that they are easily melted and converted into vapour with a moderate heat. Another objection is, that the absence of inflammability is mentioned as a characteristic; whereas many salts by a vigorous test show signs of inflammability. Muriat of ammonia, as well as the pure salt of that name, will deflagrate.-But it must be remembered that hydrogen, which is a substance highly combustible, is one of the component parts of ammonia. This then is an imperfection in the definition not easy to be surmounted; but where objects are numerous and diversified, it is difficult to class them. It is obvious from what was stated in the first lecture on chemistry, (Lect. XXV.) that all salts are compound bodies. The acids all consist of certain bases or radical principles which give the particular character to the salt, and the acidifying principle, or oxygen. Of the three alkalies, the volatile or ammonia is a compound of hydrogen and nitrogen; and soda and potass have been lately discovered to be metallic oxides, as stated in a preceding lecture. The most simple state of salts is a mass, white, brittle, and in some degree transparent. Salts in certain degrees of heat are fluid, like oil, and transparent; when cooled they return to their former state, and are semi-opake. They differ in their degrees of volatility and fusibility: some fly into vapour with the least heat, others in a violent heat remain nearly fixed; most of them require to be heated hed-hot, yet all may be brought into fusion. All saline substances dissolve more or less readily in water. The attraction of salts for water is however different in different salts. Those that have the greatest attraction for water require least for their solution: some of them possess so great an attraction for water, as to absorb it from the air. These are called deliquescent. The first phænomenon in the mixture of salts with water is the separation of air from the water by a seemingly elective attraction. This appears by a muddiness which proceeds through all the parts of the fluid, and is occasioned by a number of little bubbles, which rise to the top so as to form a scum; when all these are risen, the water becomes transparent. This deserves to be taken notice of, as many have been deceived by it, especially those who have written on mineral waters. They often mention effervescence in them, where there was none, and the appearance of it was nothing more than the air escaping. Another phænomenon is the alteration in the heat of the mixture. In some cases it is colder, When heat has been proin others hotter. duced, it has been thought owing to the violent attraction between the salt and water; for the production of cold there has been no theory of fered but that of latent heat, as stated in a preceding lecture. Another phænomenon is, that if we add more and more salt to the water, it will be more and more slowly dissolved, and after a certain quantity it will dissolve no more. When the water will dissolve no more, the point at which the salts cease to dissolve is called, as was before mentioned, the point of saturation. The proportion of water is very different with respect to different salts: some require only a quantity of water equal to themselves in weight. A saturated solution dissolves more salt if a little heat is thrown into it. Common salt is the only exception to this general rule; it dissolves with equal ease in the same quantity of cold water as in warm. Mr. Macquer thought the deliquescent salts were also exceptions; but this remark is by no means just. We are not agreed as to what salts are to be called deliquescent, and what not: their deliquescence may depend sometimes on the particular dampness of the air, and sometimes on their own particular condition. From the phænomena attending the dissolution of salt Sir Isaac Newton thought that there was an equal distribution of it through a determined space of water, and hence the deposition is in a regular order; hence also, if a space of water is increased by expansion, it is capable of arranging a greater number of particles of salt than it otherwise would do. That something of this kind really takes place is evident from experiment, as that part of the water which contains the least salt will have the greatest attraction for and thus the distribution of the salt will pro it ; ceed till the particles are arranged at equal distances through the whole fluid. There is, however, a seeming objection to this, viz. that when the salt is merely thrown into the water, the particles do not rise and disperse themselves immediately, but it is a long time before the salt is dispersed through the whole. This, though a seeming, is not a real objection. It is true, it is a long time before the salt dissolves, but this is in consequence of the very cause on which distribution depends; and in time the salt will be equally distributed through the whole. Throw a heavy salt into a glass of water (sulphat of copper, or blue vitriol, for instance), it at first sinks to the bottom, and after some days begins to impart some of its colour and qualities to the particles of water immediately surrounding it. As the water in contact only acts upon the salt, it is soon saturated, and, being thus made heavier, remains round the salt in the state of an atmosphere. The whole of the attraction of the water, then, can only act on this surrounding atmosphere, as chemical attraction reaches to so small a distance. In a little time another stratum will be formed, containing less salt than the former. Innumerable horizontal strata will at length be formed containing less salt: hence the slow diffusion if not assisted with agitation. The sulphuric acid is used in bleaching, by dilution with the water in which the linen is steeped. The bleachers at first thought it was sufficient |
