HYDROFLUORIC ACID (Formula, HF; molecular weight, zo; synonyms, fluorhydric acid, hydrogen fluoride acid obtainable either in liquid or gaseous form. It is prepared by decomposing calcium fluoride by sulphuric acid, and by heating acid potassium fluoride.
Its chief use is in etching glass, as it decomposes and dissolves silicates. Glass, being a complex silicate, is readily attacked by the acid, tetrafluoride being formed. The liquid acid etches glass bright, and is used for producing bright designs on ground glass. The gaseous acid etches It with a dull appearance, and is used for producing matt designs upon bright-surfaced glass. The glass is covered with a thin coating of wax, and the design traced through the wax down to the glass with a sharp-pointed instrument. It is then treated with the acid, which etches the exposed parts. By this means the names of the contents of bottles can be written on the glass, the same being perfectly ineffaceable.
This action of the acid upon glass causes it to be used in photography for stripping gelatine films from the glass. When working with it, however, it must be remembered that porcelain or glass dishes cannot be safely used unless coated with beeswax. Dishes made of guttapercha will resist the strongest acid. Vessels to contain the acid are also made of this substance, as glass bottles cannot be employed.
HYDROGEN.—An important elementary substance, one of the elements of water, and con tained in all vegetable and animal products. It may be obtained by passing the vapor of water over red hot iron filings. The change that takes place may thus be represented + H, It may also be produced by submitting water to the action of an electric current, by which the elements, hydrogen and oxygen, of which it is composed, are separated. Pure hydrogen is a colorless gas, without taste or odor. It burns readily in the air, combining with the oxygen to form water. Its flame is very slightly luminous, but of intense heat.
Perhaps the simplest method of preparing hydrogen gas in small quantities is given in the following description:* Hydrogen gas is generated by the action of dilute sulphuric acid on metal. Zinc is the best for our purpose.
The generator consists of a vessel (of some material not readily affected by the acid) having suspended inside of it a bell, the lower large end of which surrounds a lead cup. The lead cup holds the pieces of zinc.
We prefer to use glass vessels ordinarily. The outer vessel is covered with a brass cap,
through the center of which passes a tube. To the inner end of the tube the small end of the bell is cemented, and is by that means suspended inside the outer vessel. By the outer end of the tube connection by means of rubber hose is made with the purifying bottle. The hose is attached to that tube in the bottle, which extends inside nearly to the bottom. From the other tube or outlet of purifying bottle connection is made by means of another rubber hose with the bag.
The purifying bottle must contain water enough to cover at least an inch of lower end of the tube.
Nearly fill the lead cup in generator with pieces of zinc.71Free the bags and tubes from air by pressure and blowing.
Pour the dilute acid into genera tor. Close the stop cock in outlet tube and place the cover and bell. Before making the final connections allow some gas to escape as you make each con nection, that a free passage may be One part of sulphuric acid to six parts of water is about right for the solution. As this solution will be hot when it is first made it should be al lowed to cool before it is poured into the generator, simply as a precaution against breakage or damage to the generator. Of course, if no other glass or earthen vessel is convenient the solution may be made in the outer vessel of the generator and allowed to cool for an hour or two before the zinc is put in or the cover placed.
As the fumes from this operation are disagreeable, it should be done out of doors, or at least at an open window.
The heat spoken of above is generated by chemical action, and it must not be supposed that artificial heat is in any way required for the making of this gas.
Hydrogen is the lightest substance known, being about times lighter than air, and having a specific gravity of 0.693. Owing to its characteristic lightness, it is taken as the standard of unity in referring to the atomic weight of bodies. Two vclumes of hydrogen with six of air form an explosive mixture. Hydrogen is only but slightly soluble in water, nor is any other liquid capable of dissolving it in great percentage. It can be liquefied by exposing it to a pressure of 65o atmospheres and a temperature of 140 deg. C. It, however, remains liquid at 32o atmos pheres pressure, the temperature remaining the same. It unites with all other elementary gaseous bodies to form a large number of important compounds.