ed in weight one part; and one ounce, by twelve months' exposure, has been found to gain an addition of 64. (d) Perfectly pure sulphuric acid remains quite limpid during dilution. The sulphuric acid, however, commonly found in the shops, under the name of oil of vitriol, on admixture with water, deposits a white powder, in considerable quantity, consisting of various impurities, but chiefly of sulphate of lead. Berzelius has found, also, a minute quantity of titanium in sulphuric acid of English manufacture, and tellurium in acid prepared at Stockholm*. By evaporating sulphuric acid of commerce in a platinum dish, Dr. Ure has obtained from one half to three quarters of a grain in 100 of solid matter, consisting of about two parts of sulphate of potash and one of sulphate of leadt. (e) Sulphuric acid is nearly twice as heavy as water. The specific gravity of the strongest pure acid that can be obtained, is 1.850; but even this contains 19 (according to Dr. Wollaston, 18.44) per cent. of water, which appears essential to its constitution, and can only be separated by combining the acid with a base. Dr. Ure states that genuine commercial acid should not exceed 1.8485. When denser, its purity may be suspected. It has been ascertained by Mr. Dalton, that acid, of nearly the maximum strength, has its specific gravity very little altered, by adding or subtracting small portions of water. Thus acids containing 81 and 80 per cent. of acid, do not differ more than 1 in the third place of decimals; nor is the specific gravity proportionally changed by dilution till it falls as low as 1.78. The strength of the more concentrated acid may be better ascertained, by observing how much water is required, to bring it down to the specific gravity 1.78. The boiling point, also, Mr. Dalton has discovered, is a much better test of its strength; and he has constructed the following useful Table, in which account is taken of all these circumstances. * Thomson's Annals, x. 464. VOL. I-L Mr. Dalton's Table of the Quantity of real Acid in 100 Parts of Liquid Sulphuric Acid, at the Temperature 60° Fahrenheit. Acid Water. per cent. by per cent. by It has been ascertained by Dr. Ure that by adding about 24 per cent. of its weight of sulphate of potash to concentrated oil of vit riol, its specific gravity may be increased to 1.860. The only mode, therefore, of ascertaining exactly the strength of oil of vitriol is by saturating a known quantity with an alkali; and it may be assumed as sufficiently correct, that 100 grains of dry sub-carbonate of soda neutralize 92 grains of pure liquid sulphuric acid; or that 100 grains of the acid require 108, or 108.5, of the sub-carbonate for saturation. It is sometimes of importance to the chemical artist to know the proportion, not of real acid, but of acid of commerce, in diluted sulphuric acid of different specific gravities. An approximation to the true proportion may be obtained, by increasing the numbers, indicating the real acid, one fourth. For example, acid of the specific gravity 1.200, contains according to the above table, 20 per cent of real acid; which increased one fourth, gives 25 per cent. of acid of sp. gr. 1.849. A very copious Table of the quantities of sulphuric acid of commerce in acid of different densities, constructed by Mr. Parks from actual experiment, is given in the 40th volume of the Philosophical Magazine, and in vol. ii. of his Chemical Essays, p. 144. Its length only prevents me from inserting it here. The shorter Table of Vauquelin, in the 30th volume of Nicholson's Journal, is rendered less fit for the English chemist, because the acid, employed in the experiments on which it is founded, is inferior in density to the average acid sold in this country. In taking the specific gravity of sulphuric acid, it is of importance to attend to its temperature*, which must be examined by a thermometer, having its bulb perfectly dry. According to Dr. Ure, 10° Fahrenheit make a difference in the density of oil of vitriol of 0.005. With due attention to this, and other necessary precautions, Dr. Ure has constructed a Table, which, as it is of moderate length, and exhibits at one view the proportion not only of real or dry acid, but of liquid acid, in sulphuric acid of different specific gravities, I shall insert in the Appendix. In the memoir of which the Table forms a part, Dr. Ure has endeavoured to establish some general formulæ for calculating the proportion of oil of vitriol in dilute acid of any specific gravity, and also for finding the specific gravity corresponding to a given proportion of acidt. (f) Sulphuric acid, by a sufficient reduction of its temperature, may be frozen; and under favourable circumstances, it assumes a regular crystalline form, a considerable degree of solidity or hardness, and a density exceeding that which it possessed in a fluid state. From the experiments of Mr. Keirt it follows that there is a certain point of specific gravity (viz. 1780 to 1000), at which the sulphuric acid most readily congeals; and when of this degree of strength it requires even a less degree of cold than is sufficient to freeze water, its congelation taking place at 45° Fahrenheit. From the specific gravity of 1786 on the one hand to 1775 on the other, it freezes at 32° Fahrenheit. It is singular that it remains congealed at a temperature higher than that originally required for freezing it. Acid, for example, which did not become solid till its temperature was reduced to 32°, remains at frozen 45°. When of the specific gravity of 1843, or as nearly as possible of that of commerce, it was found by Mr. Macnabs to freeze at -15° Fahrenheit; but this acid, mixed with rather more than half its weight of water required for congelation the temperature of 36° Fahrenheit. (g) To purify sulphuric acid, it must be distilled in a glass retort, placed in the sand-bed of a reverberatory furnace. This process is a difficult one. But to those who have sufficient experience in chemical operations, the following instructions may be useful; especially as it is indispensable, in all experiments of research, to employ an acid purified by distillation. The furnace, in which this process is conducted, should have a contrivance for supporting a sand bath within it at a proper height; and an opening in the side, for transmitting the neck of the retort. * See Parke's Essays, ii. 461. Journal of Science, iv. 127. § Ibid. lxxvi. 241. (Pl. vii. fig. 62, 63.) The retort must be coated with clay and sand over its whole body, and also over that part of the neck which is exposed to the fire. It is then to be placed, the coating being previously dry, in the sand-bath, about one half filled with sulphuric acid; and a receiver must be applied, but not luted on. The fire must now be lighted, and raised with extreme caution. The first portion that comes over, amounting to about one sixth, consists chiefly of water, and may be rejected. This is followed by the concentrated acid and, at this period, there is great risk that the neck of the retort will be broken, by the contact of the condensed acid, which has a very high temperature, and which frequently cracks the glass, as effectually as the application of a red-hot iron. The fire must be regulated by the register door of the ash-pit, so that several seconds may elapse between the fall of the drops into the receiver. The process may be continued as long as any acid is condensed. The retorts, employed for this purpose, should be most attentively annealed. The difficulty of rectifying sulphuric acid is much diminished, by using a retort of the capacity of from two to four quarts when a pint of the acid is employed, and by connecting its neck with the receiver by means of an adopter three or four feet long. The retort may be set over a charcoal fire, and the flame made to play gently on its bottom. No luting is to be employed, and the receiver is to be surrounded with cold water. With this arrangement, and a cautious regulation of the heat, Dr. Ure finds that sulphuric acid may be distilled without much risk, in a continuous gentle stream*. Sulphuric acid may be less perfectly purified by diluting it with an equal weight of water, allowing the impurities to settle, decanting the clear liquor, and evaporating it to the proper degree in a glass retort. (h) The proportion of the elements of sulphuric acid has been investigated by several chemists. Berthollet oxygenated 17.846 parts of sulphur by nitric acid, and obtained a quantity of sulphuric acid, which gave 127.515 parts of sulphate of barytes. Hence 100 parts of sulphur would have formed 230.79 parts of real sulphuric acid (= about 292 of density 1.85); but this product falls short of what ought to have resulted. Klaproth, Richter, and Bucholz obtained results nearly agreeing with each other. Berzelius, to avoid all fallacy from the hydrogen contained in sulphur, combined it, in the first place, with lead, which, like other metals, always evolves much hydrogen, and then oxygenated the sulphuret. The following Table exhibits the proportions, deduced from different experiments, in 100 parts of real acid: From the experiments of Berthollet Klaproth Sulphur. Oxygen. 43.28.. 56,72 Proportions admitted by Dr. Wollaston .. 40.0 .. 60.0 If the proportions be taken at 40 sulphur and 60 oxygen, and if he acid consists, as Mr. Dalton supposes, of 1 atom of sulphur and * Journal of Science, iv. 116. S atoms of oxygen, the atom of sulphur will weigh 15; for as (603-) 20 is to 40 so is 7.5 to 15; and the weight of an atom of sulphuric acid will be 37.5. Mr. Dalton's numbers are 13 for the atom of sulphur, and 34 for that of sulphuric acid; the difference being occasioned by his taking oxygen at 7, instead of 7.5. A coincidence has been pointed out by Berzelius, which is very remarkable, and is deemed by him sufficiently general, to be admitted as a law; viz. that in any combination of two oxygenated bodies, with each other, the oxygen of the one is either a multiple or divisor of that of the other, by some simple number. Sulphuric acid, of 1850 density, affords an illustration of this principle; for it consists of 81 real acid and 19 water; and it will be found that the oxygen in the acid is, as nearly as possible, 48; and the oxygen in the water 16, so that in this case the multiple is 3, for 16 × 3 48. Various other examples of the same general principle will be given, in treating of metallic oxides. In all neutral compounds of sulphuric acid with alkaline, earthy or metallic bases, the acid contains a quantity of oxygen, which exceeds that in the base by the same multiple 3. = (i) Sulphuric acid is decomposed at the temperature of the atmosphere, by inflammable substances, and acquires a dark colour. The addition of a little brown sugar, or a drop of olive oil, to a portion of the acid, imparts to it a brownish hue, which in time changes to black. Hence this acid should always be kept in bottles with glass stoppers; for a small bit of straw or cork, if dropped into a considerable quantity of sulphuric acid, changes it in the manner that has been pointed out. (k) In high temperatures, sulphuric acid is still farther decomposed by combustible bodies. 1. Hydrogen gas, brought into contact with sulphuric acid, in a state approaching ignition, decomposes it, and water and sulphurous acid are formed. This, however, is a most dangerous and difficult process, which it is not adviseable to repeat. 2. According to Gay Lussac, sulphuric acid is decomposed by heat alone, and is resolved into two parts by measure of sulphurous acid gas, and one of oxygen gas. This experiment is best performed by passing the acid through a red-hot tube of glass or porcelain. 3. Sulphur, by being boiled in sulphuric acid, partly de-oxygenates it, and converts a portion of it into sulphurous acid, which comes over in a gaseous state. 4. Into a glass retort, put such a quantity of sulphuric acid as will fill about one fourth part of it, and add a small portion of powdered charcoal. On applying the heat of a lamp, gas will be produced very abundantly. Let this gas be conveyed by a tube fixed to the mouth of a retort, and bent in the proper manner, into an inverted jar of water; or, if it can be had, into an inverted jar of quicksilver in a mercurial apparatus. During this operation, the carbon attracts part of the oxygen of the sulphuric acid, and forms carbonic acid gas. But the sulphur is not entirely disoxygenated; and a compound is therefore formed of sulphur and oxygen, containing less oxygen than the sulphuric acid. This compound exists in the state of a gas, and its properties may next be examined. To avoid, however, the |