Читать книгу The Art of Lead Burning - C. H. Fay - Страница 6
ОглавлениеCHAPTER III.
HYDROGEN GAS.
This element was discovered by Cavendish in 1766, and was called by him inflammable air. The name hydrogen is derived from two Greek words, one signifying “water” and the other “to generate,” on account of its forming water when burnt. It occurs in its free state in the bases of volcanoes, and by the aid of the spectroscope has been detected in the sun and stars. It chiefly exists in combination with oxygen as water, and is an important constituent of all vegetable and animal substances.
Hydrogen is obtained by the decomposition of water in various ways. On a large scale, nearly pure hydrogen may be prepared by passing steam over charcoal, or coke, heated to a dull redness. If the temperature be kept sufficiently low hydrogen and carbon dioxide will be the sole products, and the latter may be removed by causing it to traverse a vessel filled with slaked lime, but if the temperature be allowed to rise too high, or an excess of air be admitted, carbon monoxide is also produced, and cannot be removed from the mixture.
Pure hydrogen is a colorless, odorless, transparent and tasteless gas, and has never been liquefied. It is very slightly soluble in water. It is the lightest of all known bodies and is not poisonous, although it cannot support life, and if mixed with a certain proportion of oxygen it can be breathed for a considerable length of time without inconvenience. It is highly inflammable, and burns in the air with an almost colorless, nonluminous flame, forming water. A burning taper is extinguished when plunged into hydrogen, and all bodies which burn in the air are incapable of burning in hydrogen.
Hydrogen does not spontaneously enter into reaction with any of the elements, although it has a powerful affinity for several of them. Thus, when hydrogen and oxygen are mixed nothing occurs, but if a lighted splint is introduced a violent explosion ensues, water being produced. Similarly chlorine and hydrogen are without action upon each other in the dark, but if the mixture is exposed to a bright light, or if heated by the passage of an electric spark, the gases are at once combined with explosive violence, forming hydrochloric acid.
Hydrogen is usually prepared by the action of zinc or iron on a solution of hydrochloric or sulphuric acid. All metals which decompose water when heated readily furnish hydrogen, on treatment with hydrochloric or sulphuric acid. Many other metals enter more or less readily (although none so readily) into reaction with these acids. Also, many other acids than sulphuric or hydrochloric acids may be used, but none acts so quickly. In all cases the action consists of the displacement of the hydrogen of the acid by the metal employed, and if the acid is not one which can enter into reaction with the displaced hydrogen, the latter is also evolved as gas.
If pure gas is required it is necessary to employ pure zinc or iron, as the impurities in the ordinary metal communicate an extremely disagreeable odor to the gas.
The pure gas is not absolutely essential for lead burning, and owing to their being much cheaper, and also on account of their increased quickness of action, the commercial qualities of sulphuric acid and zinc are employed in the generator described.
The commercial zinc is known as spelter and is sold in pigs or blocks, which are easily broken into fragments, like stove coal, with a heavy hammer. The commercial sulphuric acid is known as oil of vitriol and is sold by the pound. The acid cannot be employed in its pure state, but must be reduced with water in the proportion of one part of acid to seven parts of warm water. They must be mixed by adding the acid slowly to the water; never the water to the acid. The combination of acid and water enters at once into reaction and always generates heat, and the result of adding water to acid would be small explosions. There would be danger of the acid flying on one's clothes or into the eyes. The mixture should never be stronger than six parts of water to one of acid.
The beginner will observe from the above that the generator cannot be crowded by making the acid solution strong. Hydrogen is a peculiar gas and also a dangerous one for one ignorant of its peculiarities to experiment with, and in order to thoroughly understand it the following experiments should be demonstrated, which can be done with little expense. The beginner should note the result of each experiment as demonstrated, and carefully commit the same to memory for future reference.
Experiment 1.
Test for Hydrogen.—Fill a small jar or wide mouthed bottle with hydrogen. This is done by first filling the bottle with water, inserting the end of the tube from the hydrogen generator, having first exhausted the air in the tube, then quickly inverting the bottle and placing the neck, Fig. 2, in a pan of water (A); the water will stay in the bottle. Now turn on the hydrogen. The gas, being lighter than water, will rise to the top of the bottle (B), drive out the water, and replace it with pure hydrogen, which should be free from air. Remove the bottle from the pan of water, keeping it inverted. Thrust a lighted splint into the bottle. The gas will light and burn at the mouth of the bottle. If the splint is thrust far into the bottle it will go out. Drops of water collect in the bottle. Burning is a union with oxygen; therefore, the burning of the hydrogen shows that it has an affinity for oxygen. The splint goes out because the hydrogen does not support combustion. If no air is allowed to get into it the gas cannot burn or explode.
Fig. 2.—Experiment No. 1.
Experiment 2.
Fig. 3.—Experiment No. 2.
Proving That Hydrogen is Lighter Than Air.—Bring an inverted bottle of hydrogen close to an empty bottle, also inverted, Fig. 2. Gradually tip the bottle containing hydrogen (A) until it is brought to an upright position beneath the empty bottle. Test the bottles for hydrogen. The hydrogen will be found in the bottle (B) that was at first empty, proving that hydrogen is lighter than air, as it has risen in the empty bottle, displacing the air that was in it. If the bottle of hydrogen is left in an upright position without a cover for a few moments the gas will entirely disappear.
Experiment 3.
Fig. 4.—Experiment No. 3.
The Effect of Mixing Hydrogen and Air.—Half fill a bottle with water and invert it in a pan of water, Fig. 3, leaving the upper half filled with air. Displace the air in the bottle with hydrogen, then thrust a lighted splint into the bottle, and the gas will light with an explosion. When the bottle was half filled with water the other half was air. The hydrogen took the place of the water, so that the bottle contained equal quantities of hydrogen and air. When the hydrogen was lighted it combined with the oxygen in the air. The union of the two gases caused the explosion, proving that the combined gases are very explosive.
Experiment 4.
To Make Hydrogen From Water.—Drop a piece of potassium into a little water and cover it. The potassium floats on the water and soon burns. Potassium acts vigorously on cold water, setting free hydrogen, and unites with parts of it to form “caustic potash.”
