«THE THEORY OF IONIZATION; IONIZATION AND ELECTRICAL CONDUCTIVITY» Of the laws and hypotheses concerning gases, the one that is perhaps of most importance to chemistry is Avogadro's hypothesis. With the aid of this hypothesis, we are able to determine the relative molecular weights[49] of such elements and compounds as are gases, or are volatile at higher temperatures. If equal volumes of gases, under the same conditions of temperature and pressure, contain the same number of molecules, then the weights of such equal volumes also represent the relative weights of the molecules composing the gases. As a standard, for expressing the relative molecular weights in definite numbers, the molecular weight of oxygen is taken by convention to be 32, and all other molecular weights are expressed in terms of this standard. The density, or weight of one liter of oxygen at 0° and 760 mm., is 1.429 grams, and the ‹molecular weight expressed in grams (molar weight)› of oxygen, 32 grams, occupies, therefore, 32 / 1.429, or 22.4 liters. The weights of ‹this same volume›, 22.4 liters, of gases and vapors, calculated for 0° and 760 mm. pressure,[50] express then directly, in terms of the oxygen standard, the relative molecular weights of the elements or compounds forming the gases. The weights themselves give us directly their gram-molecular or molar weights. When molecular weights are determined in this way, with the aid of Avogadro's hypothesis, results are obtained which agree [p034] perfectly with the chemical behavior of the compounds or elements in question. The molecular weights of hydrogen chloride, water, ammonia, and marsh gas, for instance, are found to be 36.5, 18, 17 and 16, respectively, corresponding to the formulæ[51] HCl, H_{2}O, NH_{3} and CH_{4}, and in confirmation of these results we find, by methods used especially in organic chemistry, that these compounds show a chemical behavior agreeing perfectly with the presence of one, two, three and four hydrogen atoms, respectively, in their molecules. Marsh gas, for instance, by treatment with chlorine, yields a monochloride, CH_{3}Cl, a dichloride, CH_{2}Cl_{2}, a trichloride (chloroform), CHCl_{3}, and a tetrachloride, CCl_{4}. Water, by proper treatment, may be converted in successive stages into alcohol, (C_{2}H_{5})OH, and then into ether, (C_{2}H_{5})O(C_{2}H_{5}), or into sodium hydroxide, NaOH, and sodium oxide, Na_{2}O.