--It is the lightest of the elements, 14 1/2 times as light asair. It occurs uncombined in coal-mines, and some other places, but the readiness with which it unites with other elements, particularly O, prevents its accumulation in large quantities. It constitutes two-thirds of the volume of the gases resulting from the decomposition of water, and one-ninth of the weight. Compute the latter from its symbol. It is a constituent of plants and animals, and some rocks. Considering the volume of the ocean, the total amount of H is large. It can be separated from H2O by electrolysis, or by C, as in the manufacture of water gas. When burned with O it forms H2O. Pure O and H when burning give great heat, but little light. The oxy-hydrogen blow-pipe (Fig. 17) is a device for producing the highest temperatures of combustion. It has O in the inner tube and H in the outer. Why would it not be better the other way? These unite at the end, and are burned, giving great heat. A piece of lime put into the flame gives the brilliant Drummond or calcium light. Chapter IX. UNION BY WEIGHT. 39. In the Equation -- Zn + 2 HCl = ZnCl2 + 2 H 65 + 73 = 136 + 2 65 parts by weight of Zn are required to liberate 2 parts by weight of H; or, by using 65 g Zn with 73 g HCl, we obtain 2 g H. If twice as much Zn (130 g) were used, 4 g H could be obtained, with, of course, twice as much HCl. With 260 g. Zn, how much H could be liberated? A proportion may be made as follows:-- Zn given : Zn required :: H given : H required. 65 : 260 :: 2 : x. [footnote: Given, as here used, means the weight called for by the equation; required means that called for by the question.] Solving, we have 8 g H. How much H is obtainable by using 5 g Zn, as in the experiment? To avoid error in solving similar problems, the best plan is as follows:-- Zn + 2HCl = ZnCl2 + 2 H | 65:5::2:x 65 2 | 65 x = 10 5 x | x = 10/65 = 2/13 Ans. 2/13 g. The equation should first be written; next, the atomic or molecular weights which you wish to use, and only those, to avoid confusion; then, on the third line, the quantity of the substance to be used, with underneath the substance wanted. The example above will best how this. This plan will prevent the possibility of error. The proportion will then be:-- a given : a required :: b given : b required. How much Zn is required to produce 30 g. H? Zn + 2HCl = ZnCl2 + 2H | 2:30::65:x 65 2 | 2x = 1950 x 30 | x = 975 Ans. 975 g. Zn. Solve:-- (1) How much Zn is necessary for 14 g. H? (2) How many pounds of Zn are necessary for 3 pounds of H? (3) How many grams of H from 17 g. of Zn? (4) How many tons of H from 1/2 ton of Zn? Suppose we wish to find how much chlorhydric acid--pure gas-- will give 12 g. H. The question involves only HCl and H. Arrange as follows:--