FLUORHYDRIC ACID.

=

CaFl2+ H2SO, CaSO, + 2HF1.

4

137

Since the acid rapidly corrodes glass, the process must be conducted in metallic vessels. Ordinarily, retorts of lead or platinum are employed, and the distillate is collected in receivers made of the same metals, and carefully cooled by means of ice.

157. The product of the distillation is a very volatile, colorless liquid, a little heavier than water. It is strongly acid, emits copious white and highly suffocating fumes in the air, boils at 15°, and remains unfrozen at -20°. On account of its corrosive power, this substance is highly dangerous; if any of it happens to come in contact with the skin, wounds are produced which are very difficult to heal; a single drop of it is sufficient to occasion a deep and painful sore. In preparing the acid, special provision must be made for carrying away from the operator any fumes which may escape condensation.

The acid may be kept in bottles made of lead or silver, or of gutta percha, substances upon which it has no action. It unites with water with great avidity, so much heat being evolved that a hissing noise is produced, as if a bar of red-hot iron had been immersed in the water. In its concentrated form the acid has a specific gravity of 1.061, but on the addition of a certain amount of water the density increases to 1.15, a definite hydrate (HF1+2H,O) being formed, which boils at 120°, and may be distilled unchanged. The further addition of water to this hydrate is attended with a regular decrease in density.

According to some chemists, the liquid acid obtained as above described is not anhydrous. It is asserted that if it be distilled with an excess of anhydrous phosphoric acid (a substance which has a very strong affinity for water), the anhydride will be set free in the form of a colorless, extremely irritating gas.

158. Upon metals and metallic oxides, fluorhydric acid acts. like chlorhydric acid, only more powerfully; but its most striking peculiarity is its action upon silica and the compounds of silica, such as glass or porcelain. If a drop of the concentrated acid be allowed to fall upon a piece of glass, it becomes hot, boils, and partially distils off as a fluoride of silicon, while the glass is corroded and becomes covered with a white powder consisting of compounds of fluorine and various constituents of the glass. If this

138

ETCHING BY FLUORHYDRIC ACID.

powder be washed away a deep impression will be found upon the glass at the point where the acid has acted.

This corrosive power, which is possessed by fluorhydric acid gas as well as its aqueous solution, is made use of for etching glass. The graduations on the glass stems of thermometers and eudiometers may thus be made with great precision and facility; the acid is largely employed also in ornamenting glass with etched patterns.

Exp. 74.-Warm a slip of glass and rub it with beeswax so that it shall be everywhere covered with a thin, uniform layer of the wax. With a needle, or other pointed instrument, write a name, or trace any outline through the wax, so as to expose a portion of the glass. Lay the etching, face downward, upon a bowl or trough of sheet-lead, in which has been placed a teaspoonful of powdered fluor-spar and enough strong sulphuric acid to convert it into a thin paste; if the glass be smaller than the opening of the dish, it may be supported upon wires laid across the latter.

Cover the glass and the top of the dish with a sheet of paper, and then gently heat the leaden vessel for a few moments, taking care not to melt the wax; then set the dish aside in a warm place and leave it at rest during an hour or two. Finally melt the wax and wipe it off the glass with a towel or bit of paper; the glass will be found to be etched and corroded at the places where it was laid bare by the removal of the wax.

This experiment can be performed more rapidly by covering the outside of a watch-glass with wax, tracing characters upon this layer, and then placing the glass upon a small platinum crucible containing a mixture of fluor-spar and sulphuric acid, which is heated over the gaslamp. The watch-glass is meanwhile kept full of water, in order to prevent the wax from melting. In this way the etching can be effected in the course of a few minutes.

Instead of the gas, a dilute aqueous solution of the acid may be employed in this experiment. The concentrated acid of § 157, diluted with six parts of water, answers a good purpose. In this case the etched surface will appear smooth like the rest of the glass, while in case the gas is employed the etched portion of the glass will be dull and rough.

159. No compounds of fluorine with chlorine, bromine, iodine, nitrogen, or oxygen have yet been discovered, though a sulphur compound has been obtained, as a fuming liquid, by distilling fluoride of lead with sulphur. Fluorine is the only element of

which no oxygen compound is known; this fact, however, will appear less remarkable if it be remembered that, in order to obtain oxygen compounds of chlorine, bromine, and iodine, it is necessary first to isolate these elements, and to have them in the free and uncombined condition. Analogy would therefore teach that a practicable method of preparing free fluorine must be discovered before we can hope to prepare oxides of fluorine.

160. The fact that fluorine forms a powerful acid with hydrogen, connects this element with the three elements (chlorine, bromine, and iodine) which have last been studied. Many of its compounds with the metals are analogous in composition to the compounds of chlorine, bromine, and iodine, and not a few of these compounds are isomorphous with one another. It is customary therefore to study fluorine in connexion with the chlorine group; but the student should remember that in several respects it differs widely from chlorine, and that its connexion therewith is, in any event, less intimate than that of either bromine or iodine.

161. Besides ordinary oxygen, such as is found in the air and has been prepared in Exps. 5 and 7, two other kinds or forms of this element are known to chemists. These new modifications of oxygen have received special names, and are called ozone and antozone respectively.

162. Several other elements, notably sulphur, phosphorus, and carbon, occur, as oxygen does, in very unlike states, or with very different attributes, while the fundamental chemical identity of the substance is preserved. The word allotropism is employed to express this capability of some of the elements; it is derived from Greek words signifying of a different habit, or character. This word serves merely to bring into one category a considerable number of conspicuous facts, of whose essential nature we have

no knowledge; there is, of course, no virtue in the word itself to explain or account for the phenomena to which it refers.

163. Ozone is an exceedingly energetic chemical agent, which resembles chlorine in some respects; it can therefore be advantageously studied in connexion with the chlorine group. Moreover, since ozone and antozone were for a long time confounded with one another, and since they are really intimately related, they should, of course, be studied together. The most natural connexion of these somewhat obscure bodies is with oxygen; but we are better able to appreciate what is known of the properties of ozone and antozone now that we have become acquainted with a number of the elements, and have made ourselves familiar with a considerable variety of chemical processes and reactions, than we were at the very outset, when common oxygen was necessarily studied.

164. It had long been noticed that when an electrical machine was put in operation a peculiar, pungent odor was developed; but it is only at a comparatively recent period that it has been observed that the same odor is manifested during the electrolysis of water (§ 35), and that this odor resembles that evolved by moistened phosphorus when exposed to the air. It has gradually been made out that the odor in each of these cases is due to the presence of a peculiar modification of oxygen, called ozone from a Greek word signifying to smell. This modification of oxygen was at one time erroneously supposed by some to be a high oxide of hydrogen, of composition H,O,, or H,O,; but this view has lately been completely disproved.

2

Of the methods of obtaining ozone above suggested, that by phosphorus will usually be found most convenient.

Exp. 75.-In a clean bottle, of 1 or 2 litres capacity, place a piece of phosphorus 2 or 3 c.m. long, the surface of which has been scraped clean (under water) with a knife; pour water into the bottle until the phosphorus is half covered; close the bottle with a loose stopper, and set it aside in a place where the temperature is 20° or 30°.

In the course of ten or fifteen minutes a column of fog will be seen to rise from that portion of the phosphorus which projects above the water, the original garlic odor of the phosphorus will soon be lost, and the peculiar odor of ozone will gradually pervade the bottle. After

five or six hours, the bottle will be found to contain an abundance of ozone for use in the subsequent experiments.

The chemical changes which occur during this experiment are complicated; it will be enough to say of them that the phosphorus unites with oxygen from the air in the bottle to form an oxide of phosphorus, which will be studied hereafter under the name of phosphorous acid; that during this process of oxidation a portion of the oxygen in the bottle is changed into ozone and antozone, and that some of the ozone remains, even after many hours, diffused in the air of the bottle.

165. It must be distinctly understood that no very large quantity of ozone is obtained in the foregoing experiment. At the best, only a very minute proportion of it will be found in the air of the bottle. But ozone is a substance possessing great chemical power, and but little of it is needed in order to exhibit its characteristic properties.

If it be desired to prepare ozone by passing electric discharges through air or oxygen, either of these gases may be sealed up in narrow glass tubes, through the centres of which are passed platinum wires, welded tightly into the glass, as shown in Fig. 37, and a series of sparks from an electrical machine is thrown through the gas in the tube, during ten or twelve hours. If the experiment be continued longer than this, nothing is gained; for the sparks after this time appear to destroy the ozone previously produced.

To avoid the difficulty last named, a slow current of oxygen may be forced through a tube open at both ends, and electrical discharges may be passed through the gas in its transit; a constant stream of ozonized air will be thus obtained.

Instead of the sparks, the gas within the tube may be subjected to silent discharges of electricity obtained by conFig. 37. necting one of the platinum wires with the ground, the other with the prime conductor of an electrical machine, and slowly turning the crank of the latter. By using a tube having wires near the top, as in Fig. 37, and closing the lower end of the tube by immersing it in a bath filled with an aqueous solution of iodide of potassium, so that the ozone may be absorbed as fast as it is formed, it has been found possible, by some experimenters, to transform and remove all the original oxygen contained in the tube.

166. Ozone is produced not only during the slow oxidation of