escape in combination with the fluoric acid; and the alkali, if any be present, may be obtained in the state of sulphate by digesting the residual matter with water. Class L Div. I. Silica has the property of combining in definite proportions Hydrates, with water. But the compound is not very intimate. Yet that an affinity exists between silica and water is obvious from the gelatinous state which it assumes. Gelatinous silica allowed to dry in the open air till it ceases to wet paper, though still retaining its state, is a compound of When the jelly is allowed to dry in the the appearance of gum arabic, or indeed of chalcedony (only it wants the hardness of that mineral). In that state it is a compound of 1 atom silica 1 atom water When silica is fused with an alkaline carbonate, if we dissolve 4 1.125 5.125* Though silica does not redden vegetable blues, yet it enters into definite compounds with the different bases, and forms saline compounds which are distinguished by the name of sili Like other weak acids, it is capable of entering into a great variety of combinations with bases. The compounds which it forms are frequently so intimate, that no other acid is capable of removing the base and setting the silica at liberty. The different kinds of glass consist of two or more silicates melted together. Silicates of potash and of soda are soluble in water, but when a little of an earthy silicate is added the glass becomes insoluble. I have shown in the first volume of this work (p. 225), that the atomic weight of silica is 2, and that it is a compound of * First Principles, i. 191. Chap. I. 1 atom silicon 1 atom oxygen 2 Preparation. Hydrates. SECTION VI.-OF THE ACIDS OF PHOSPHORUS. Phosphorus combines in various proportions with oxygen, and forms at least four acids, of which I shall treat in the present section. 1. Phosphoric Acid. This acid exists chiefly in the animal kingdom, where united to lime it constitutes about #ths of the earth of bones. It occurs also in the mineral kingdom, constituting 13 mineral species in which this acid occurs united to different bases, as lime, magnesia, alumina, oxides of iron, copper, uranium, &c. It is usually procured from earth of bones, and I have given one of the easiest processes in the first volume of this work (p. 237). We may obtain the acid likewise by burning phosphorus in a porcelain cup, and covering it with a glass jar capable of holding about 300 cubic inches. The phosphorus burns with splendour, and a white smoke rises which condenses upon the inside of the glass like flakes of snow. This combustion may be repeated (taking care to renew the air of the jar every time), till we obtain enough of phosphoric acid for the object in view. Another method (first put in practice by Lavoisier), is to dissolve phosphorus by boiling it gently in a retort with twelve times its weight of strong nitric acid, previously diluted with twice its weight of water. The solution goes on with effervescence, deutoxide of azote being evolved. A receiver should be attached to the retort, and after the phosphorus is dissolved the nitric acid should be gently distilled off, till the acid in the retort acquires the thickness of a syrup. It is now to be put into a platinum crucible and gradually raised to a red heat. It gives out water, and melts into an oily looking liquid, and by continuing the heat it becomes at last a transparent, colourless liquid like water, which, on cooling, assumes the appearance of perfectly limpid glass. In this state I found it a compound of By continuing It was therefore a hydrate of phosphoric acid. Now 0.469 of water is not much more than of an atom. Rose, therefore, had obtained nearly a trishydrate, or compound of Class I. Div. J. But however long continued the heat may be, we cannot drive off the whole of the water. Though the volatility of this acid be inconsiderable, yet at a red heat it rises in a white smoke, and might probably be dissipated, were the heat long enough continued. With the vapour of water it volatilizes very perceptibly. The specific gravity of this acid, in a state of dryness, is 2-687;* in the state of glass, 2.8516;† in the state of deliquescence, 1.417.‡ This acid is very soluble in water. When in the state of white flakes, it dissolves with a hissing noise, similar to that made by red-hot iron plunged into water. When in the state of glass it dissolves much more slowly. The heat evolved during the combination of this acid and water is much inferior to that evolved when sulphuric acid enters into a similar combination. Phosphoric acid obtained by deliquescence, when mixed with an equal quantity of distilled water, acquired so little heat as to raise the thermometer only one degree, as Mr. Sage observed. M. Lavoisier raised the thermometer from 50° to 63° by mixing phosphoric acid, boiled to the consistence of a syrup, with an equal quantity of water; and from 50° to 104° when the acid was as thick as turpentine.§ When the flocks of this acid have been ignited, they resist solution in water for a considerable time, and float about in it not unlike flocks of Chap. I. silica. Yet they at last disappear, and constitute a limpid solution, provided the phosphoric acid be quite disengaged from a base. Though this acid acts with but little energy upon animal and vegetable substances, it combines readily with bases, and forms a class of salts called phosphates. Its tendency to unite with these bases is very energetic, and when assisted by heat it is capable of driving off all acids which possess volatility, even sulphuric, nitric, and muriatic, which have a stronger affinity than it for bases by the moist way. It has the property of uniting with most bases in a variety of proportions, and the alkaline phosphates crystallize most readily when they contain two atoms of acid united to one atom of base. When dropt into a solution of nitrate of silver, a yellow precipitate falls, which is a subsalt. This tendency which it has to form subsalts, makes it very difficult to analyze the phosphates by precipitation. We can dissolve iron and zine in dilute phosphoric acid, but scarcely any of the other metals. The phosphates are best obtained by double decomposition. A great number of them are insoluble in water; but they all dissolve in nitric and muriatic acids. I have shown in the first volume of this work (p. 238), that phosphoric acid is a compound of Discovery. 2. Pyrophosphoric Acid. In the year 1827, Mr. Thomas Clark of Glasgow, observed, that when the common phosphate of soda of the shops was exposed to a red heat, its nature was altered. When the ignited salt was dissolved in water and crystallized, the shape of the crystals was quite different from that of common phosphate; it contained only half as much water of crystallization, and it was not the least altered by exposure to the air, whereas common phosphate of soda speedily effloresces. When a solution of this new salt was dropt into nitrate of silver, a white precipitate fell consisting of neutral phosphate of silver, whereas common phosphate of soda throws down a yellow-coloured subsalt. Mr. Clark gave to his new salt the name of pyrophosphate of soda, and to the acid which it contains, and which obviously possesses different properties from those of common phosphoric acid, the name of pyrophosphoric acid.* M. Stromeyer of Gottingen informs us that one of his pupils had some years ago made a similar observation with Mr. Clark, that ignited phosphate of soda precipitated silver white. And Stromeyer himself had ascertained that phosphoric acid prepared by means of nitric acid from phosphorus, acquired the same properties by ignition, and that phosphoric acid prepared by burning phosphorus possesses it also. Stromeyer resumed his investigation of the subject after the publication of Mr. Clark's paper, and ascertained that the characters and composition of the phosphate and pyrophosphate of silver are quite different. He found that when pyrophosphate of silver and phosphate of soda are boiled together a double decomposition takes place, and there are formed phosphate of silver and pyrophosphate of soda. All the other pyrophosphates tried were equally decomposed by phosphate of soda. He found also that when nitrate of silver was dropt into a mixture of phosphate and pyrophosphate of soda, phosphate of silver precipitates first and afterwards pyrophosphate of silver. It is obvious from this that pyrophosphoric acid is a weaker acid than phosphoric. The pyrophosphates of lead, copper, nickel, cobalt, uranium, bismuth, manganese, mercury, glucina, and yttria, are redissolved with great facility by an excess of pyrophosphate of soda. While the precipitates produced by phosphate of soda are not redissolved by an excess of that salt, this shows very clearly that the pyrophosphates have a much greater tendency to form double salts than the phosphates.+ It is clear from all this that pyrophosphoric and phosphoric acids possess quite distinct characters. But Stromeyer has shown that the proportion of the elements of both is exactly the same, each being a compound of 1 atom phosphorus 21 atoms oxygen I was induced at the request of Mr. Clark to examine the con- Class I. Div. L |