Nitrogen exists in many compounds, such as the nitrates; but the great source of it all is the atmosphere. See page 85. CHAPTER VIII. HYDROGEN. 31. Preparation. Experiment 20.--Prepare apparatus as for making O. Be sure that the cork perfectly fits both d.t. and t.t., or the H will escape. Cover 5 g. granulated Zn, in the t.t., with 10 cc. H2O, and add 5 cc. chlorhydric acid, HCl. Adjust as for O (Fig. 7), except that no heat is to be applied. If the action is not brisk enough, add more HCl. Collect several receivers of the gas over water, adding small quantities of HCl when necessary. Observe the black floating residuum; it is carbon, lead, etc. With a glass plate remove the receivers, keeping them inverted (Fig. 11), or the H will escape. 32. The Chemical Change is as follows:-- Zinc + hydrogen chloride = zinc chloride + hydrogen. Zn + 2 HCl = ZnCl2 + 2H. Complete by adding the weights, and explain. Notice that the water does not take part in the change; it is added to dissolve the ZnCl2 formed, and thus keep it from coating the Zn and preventing further action of the acid. Note also that Zn has simply changed places with H, one atom of the former having driven off two atoms of the latter. The H, having nothing to unite with, is set free as a gas, and collected over water. Of course Zn must have a stronger chemical affinity for Cl than H has, or the change could not have taken place. Why one Zn atom replaces two H atoms will be explained later, asfar as an explanation is possible. This equation, should be studied carefully, as a type of all equations. The left-hand member shows what were taken, i.e. the factors; the right-hand shows what were obtained, i.e. the products. H2SO4 might have been used instead of HCl. In that case the reaction, or equation, would have been: -- Zinc + hydrogen sulphate = zinc sulphate + hydrogen. Zn + H2SO4 = ZnSO4 + 2H. Iron might have been used instead of zinc, in which case the reactions would have been:-- Iron + hydrogen chloride = iron chloride + hydrogen. Fe + 2 HCl = FeCl2 + 2 H. Iron + hydrogen sulphate = iron sulphate + hydrogen. Fe + H2SO4 = FeSO4 + 2 H. Write the weights and explain the equations. The latter should be memorized. 33. Properties. Experiment 21.--Lift with the left hand a receiver of H, still inverted, and insert a burning splinter with the right (Fig. 12). Does the splinter continue to burn? Does the gas burn? If so, where? Is the light brilliant? Note the color of the flame. Is there any explosion? Try this experiment with several receivers. Is the gas a supporter of combustion? i.e. will carbon burn in it? Is it combustible? i.e. does it burn? If so, it unites with some part of the air. With what part?34. Collecting H by Upward Displacement.