Chap. 111. to chlorine, bromine, iodine, sulphur, selenium, &c., have already been described, so far as they are known, in the first volume of this work-and the salts which they form with their respective acids will come under our review hereafter. I may here close this chapter that we may enter upon an examination of the neutral compounds, which will occupy our attention in the next chapter. CHAP. III. OF NEUTRAL COMPOUNDS. I MIGHT in this chapter describe a considerable proportion of the vegetable and animal principles; for many of the most important of these principles neither possess the characters of acids or alkalies, at least so far as hitherto observed. But I shall confine myself in this place to those principles which are useful as chemical reagents, and which therefore it is of importance for the student to be acquainted with before he turns his attention to vegetable or animal chemistry. These neutral substances naturally arrange themselves under seven different heads. I shall, therefore, divide this chapter into the seven following sections: 1. Water, 5. Volatile oils, 6. Fixed oils, 2. Spirits, 3. Ether, 7. Bitumens. Dissolves bodies. 4. Ethal, SECTION I.-OF WATER. I have already, in the first volume of this work (p. 103), described the physical properties of water, and given an account of its composition. It remains only in this place to make a few observations upon it as a chemical body. 1. It has the property of dissolving and combining with almost all the acids, and with several of the alkaline bodies; and as it shows an equal disposition to combine with either, and does not destroy nor conceal their acid and alkaline qualities, it is obvious that it is a neutral body. It has the property also of dissolving a considerable number of the salts, which are compounds of an acid and a base. The quantity of each of these bodies which water can dissolve has a limit, and it is very various with respect to different salts. When the water has dissolved as much of any salt as it can take up, we say that it Sect. I. is saturated with the salt. If we add more of the salt after it is thus saturated, it will remain at the bottom of the liquid undissolved. Or if we cause it to dissolve by increasing the temperature of the water, it is again deposited in crystals when the liquid is allowed to cool. It is obvious that the power which thus limits the solvent property of water is the attraction which exists between the particles of the salt. When a salt is dissolved in water its particles must be equally dispersed through every part of the liquid. They must of course be arranged in regular rank and file, and the greater the quantity dissolved the smaller must the distance be between every two particles of the salt. It would appear that the greater the number of particles of salt which are dissolved by the water, the smaller is the force by which the salt and water are united.* solution. This may be understood by considering the particle of salt Nature of to be surrounded by a certain number of particles of water, each of which is attracted to it, and the aggregate attractions of which will represent the force, by means of which the particle of salt is kept suspended in the liquid. Thus at the temperature of 580.25, 100 parts of water are capable of dissolving 7.74 parts of anhydrous carbonate of soda. The atomic weight of anhydrous carbonate of soda being 6·75, and that of water 1.125. It is obvious that every integrant particle of the salt must be united to about 88 integrant particles of water. We may consider every particle of the salt as surrounded by 88 particles of water, each of which attracts it to itself by a certain unknown force; so that the whole liquid may be considered as composed of small spheres, each of which has a particle of salt in the centre, while the rest of the sphere is composed of 88 particles of water. Let us suppose now that by means of heat we can enable the 100 parts of water to dissolve an additional 7·74 parts of anhydrous salt. It is clear that the compound will now consist of 1 atom salt united to only 44 atoms water: the number of atoms of water united to the atom of salt will now be reduced to one-half the former number. The spheres will be smaller, and consequently the particles of salt nearer each other than in the former case. When the solution cools the particles of salt being combined with only half the atoms of water, and being nearer each other, a certain number of them See the experiments related with another object in view, in my treatise On Heat and Electricity, p. 197. Chap. III. unite and form crystals, which in consequence of their weight, are gradually deposited at the bottom of the liquid. What increases this tendency to crystallize is, that the crystal formed is not anhydrous, but a combination of 1 atom of salt and 10 atoms water. It would appear that the combination between these 10 atoms and the atom of salt is more intimate The hot solution consisted of a congeries of sphericles consisting each of an atom of anhydrous salt, surrounded by 25:32 atoms of water. About th of the anhydrous salt was deposited in crystals when the cork was drawn, each atom of which carried along with it 10 atoms of water. So that there remained a solution consisting of sphericles, in the centre of each of which is an atom of salt surrounded by 30 atoms of water, which represents the solubility of the salt at 64°. Water not only dissolves many salts and other bodies; but it has the property of entering into combination with a great many bodies in a solid state, constituting compounds, to which the name of hydrates was given by Proust; and this name, though in some respects exceptionable, has been universally adopted by chemists. There are few or none of the simple bodies which are capable of forming hydrates with water. The supporters of combustion are soluble in it to a trifling extent; and the same remark applies to hydrogen and azote; but none of the other bases, whether acid or alkaline, are capable of uniting with it. Most of the acids are capable of forming hydrates. Such hydrates are usually called crystals of the acid; sometimes they are in the state of powders; or sometimes they constitute jellies. Most of the alkaline bases in like manner constitute hydrates, some of them in crystals; but a much greater proportion in the state of dry powders. I have already in the preceding part of this work given an account of all of these hydrates hitherto examined. No farther observations, therefore, seem requisite here. SECTION II.--OF ARDENT SPIRITS. Sect. II. The term ardent spirits in this country, is usually applied to the liquid obtained by distillation from different fermented liquors; all of which, as has been long known, are nothing else than alcohol more or less diluted with water, and flavoured by means of some volatile oil or other. But of late years two distinct species of liquid have been discovered, bearing a much Species. closer resemblance to alcohol than to any other body whatever, though at the same time so different, that they must be considered apart. These are pyroacetic and pyroxylic spirits. I shall therefore subdivide this section into three parts; in the first of which I will give an account of alcohol, in the second of pyroacetic spirit, and in the third of pyroxylic spirit. I. Alcohol, Fermented liquors are of two kinds. They either consist of History. the expressed juices of various kinds of fruit, as grapes, currants, apples, &c. These juices when fermented, are known by the name of wine. Or they consist of the hot infusion of various kinds of corn, as barley, rye, rice, wheat, &c. These infusions, when fermented, are known by the names of beer and ale. Both of these kinds of liquors were known at a very early period. The Scripture informs us that Noah planted a vineyard and drank wine; and the heathen writers are unanimous in ascribing the invention of this liquor to their earliest kings and heroes. Beer, too, seems to have been discovered at a very remote period. It was in common use in Egypt during Chap. III. the time of Herodotus.* Tacitus informs us that it was the Whether the ancients had any method Rectification. drink of the Germans.† of procuring ardent spirits from these or any other liquors does beer of the Germans we do not know. At what period these liquors were first subjected to distillation is unknown; though it can scarcely have preceded the time of the alchymists. The process is simple. Nothing more is absolutely necessary than to boil them in a still. The first portion of what comes over is ardent spirits. It is certain, at least, that the method of procuring ardent spirits by distillation was known in the dark ages; and it is more than probable that it was practised in the north of Europe much earlier. They are mentioned expressly by Thaddeus, Villanovanus, and Lully.‡ It is by the distillation of fermented liquors that ardent spirits are obtained; and they receive various names according to the nature of the substance employed. Thus brandy is procured from wine, rum from the fermented juice of the sugarcane, whisky and gin from the fermented infusion of malt or grain. Now ardent spirits, whatever be their name, consist almost entirely of three ingredients: namely, water, pure spirit or alcohol, and a little oil or resin, to which they owe their flavour and colour. 1. When these spirituous liquors are redistilled, the first portion that comes over is a fine light transparent liquid, known in commerce by the name of rectified spirits, and commonly sold under the denomination of alcohol or spirit of wine. It is not, however, as strong as possible, still containing a considerable portion of water. The method usually practised to get rid of this water is to mix the spirits with a quantity of very dry and warm salt of tartar.§ This salt has a strong attraction for water, and the greatest part of it is insoluble in alcohol. It accordingly combines with the water of the spirit; and the solution thus formed sinks to the bottom of the vessel, and the alcohol, which is lighter, swims over it, and may easily be decanted off; or, what * Lib. ii. n. 77. Bergman, iv. art. ii. 4. † De Morib. Germ. ch. xxiii. |