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and of water, and the heat produced by the combustion of marsh gas, the thermal equations being
(1) C+0,=CO2 +97,000 cal.
(3) CH. +20,=CO+2H,0+212,000 cal. The difference between the thermal value of the last process and the sum of the first and second represents the heat of formation of marsh gas
97,000+ 136,800 - 212,000=21,800, hence we get the expression
C+2H2=CH. +21,800 cal.
THE STUDY OF FOUR TYPICAL ELEMENTS HYDROGEN-OXYGEN-NITROGEN-CARBON
AND THEIR MORE IMPORTANT COMPOUNDS
HYDROGEN Symbol, H. Atomic weight=1.01. Molecular weight=2.02. Density=1.01.
History.-The existence of hydrogen as an individual substance was first established by Cavendish (1766), who applied to it the name inflammable air. He obtained the gas by acting upon certain metals, as iron, tin, and zinc, with either sulphuric or hydrochloric acid.
Occurrence.- In the free state hydrogen occurs only in small quantities upon the earth. It is evolved with other volcanic gases, and is present in the gases which escape from petroleum wells. It is evolved also during the fermentation and decomposition of certain organic compounds, and is therefore present in the breath and the intestinal gases of animals. From these sources it finds its way into the atmosphere, where it is present to the extent of about .2 volumes in 1000 volumes of air. Hydrogen has also been found in many specimens of meteoric iron, and also in certain rocks, where it is present as occluded gas.
Hydrogen in the uncombined state exists in enormous masses upon the sun, and is present in certain stars and nebulæ. The so-called prominences which are seen projecting from the sun's disk to a distance of many thousands of miles, and which were
first observed during solar eclipses, consist of vast masses of incandescent hydrogen.
In combination with other elements hydrogen is extremely abundant ; its commonest compound is water, which consists of one part by weight of this element combined with eight parts of oxygen. In combination with chlorine, as hydrochloric acid, with carbon as marsh gas, and with sulphur as sulphuretted hydrogen, this element also occurs in large quantities. All known acids
contain hydrogen as one of their constituents, and it is present in almost all organic compounds.
Modes of Formation.-(1.) Hydrogen may be obtained from water by the action of various metals upon that compound under certain conditions. The metals sodium and potassium will decompose water at the ordinary temperatures ; when, therefore, a fragment of either of these metals is thrown upon water, the latter is decomposed and hydrogen set free :
The metals, being lighter than water, float upon its surface, and, owing to the heat of the reaction, inelt and roll about upon the liquid as molten globules. With potassium, the heat developed is sufficiently great to cause the hydrogen to inflame, and it burns with a flame coloured violet by the vapour of the metal. The hydroxide of the metal, which is the second product of the action, dissolves in the excess of water, rendering the liquid alkaline. The alkalinity of the solution may be made evident by the addition of a reddened solution of litmus, which will be turned blue by the alkali.
In order to collect the hydrogen evolved by the action of sodium upon water, the metal is placed in a short piece of lead tube closed at one end, which causes it to sink in the liquid, and an inverted glass cylinder filled with water is placed over it, as shown in Fig. 27. The evolved hydrogen then rises as a stream of bubbies into the cylinder and displaces the water.*
(2.) Water may be readily decomposed at the boiling-point, by means of zinc, if the metal be previously coated with a thin film of copper by immersion in a dilute solution of copper sulphate. When this copper-coated zinc (known as zinc - copper couple) is heated in a small flask filled with water, and provided with a delivery tube, the oxygen of the water combines with the zinc forming zinc oxide, and hydrogen is evolved, which may be collected over water at the pneumatic trough :*~
Zn+H,O=Zn2+H. * For detailed description of these experiments, see Newth's "Chemical Lecture Experiments," p. 2.
(3.) At a still higher temperature, water in the state of steam can be readily decomposed by the metal magnesium, magnesium oxide being formed and hydrogen liberated :
Mg+H2O=MgO + H2. For this purpose the magnesium is strongly heated in a glass bulb (Fig. 28), while steam from a small boiler is passed over it. As the temperature of the metal approaches a red heat it bursts into flame, and the issuing hydrogen may be ignited as it escapes from the end of the tube.
(4.) If iron be heated to bright redness and steam be passed over it, the water is decomposed, the oxygen uniting with the iron
to form an oxide known as triferric tetroxide, or magnetic oxide of iron, thus
3Fe+ 4H,0=Fe3O4 + 4H2. This method is employed on a large scale for the preparation of hydrogen for commercial purposes. Iron borings or turnings are packed into an iron tube, which is strongly heated in a furnace, and steam from a boiler is passed through the tube.
(5.) For laboratory purposes hydrogen is most conveniently prepared by the action of dilute sulphuric acid upon zinc :
Zn+H,80,=ZnSO4+H. For this purpose granulated zinc (i.e. zinc which has been melted