For, if the whole current which passes through a solution is carried by the ions—and if it were not we should not always find the deposits exactly proportional to atomic weights—then the ratio of the total quantity of electricity passing to the weight of the deposit produced must be the same as the ratio of the charge on each ion to the mass of that ion. But by international agreement one absolute unit of electricity has been defined in the electromagnetic system of units as the amount of electricity which will deposit from a silver solution 0.01118 grams of metallic silver. Hence if refers to the silver ion and E means the charge on the ion, we have [Pg 30] or if refers to the hydrogen ion, since the atomic weight of silver is times that of hydrogen, which is about . Thus in electrolysis varies from ion to ion, being for univalent ions, for which is the same and equal to one electron , inversely proportional to the atomic-weight of the ion. For polivalent ions may be 2, 3, 4, or 5 electrons, but since hydrogen is at least 7 times lighter than any other ion which is ever found in solution, and its charge is but one electron, we see that the largest value which ever has in electrolysis is its value for hydrogen, namely, about . Although varies with the nature of the ion, there is a quantity which can be deduced from it which is a universal constant. This quantity is denoted by , where means as before an electron and is the Avogadro constant or the number of molecules in 16 grams of oxygen, i.e., in one gram molecule. We can get this at once from the value of by letting refer to the mass of that imaginary univalent atom which is the unit of our atomic weight system, namely, an atom which is exactly as heavy as oxygen or as heavy as silver. For such an atom Multiplying both numerator and denominator by and remembering that for this gas one gram molecule means 1 gram, that is , we have [Pg 31] and since the electromagnetic unit is equivalent to , we have Further, since a gram molecule of an ideal gas under standard conditions, i.e., at 0° C. 76 cm. pressure, occupies 22412 c.c., if represents the number of molecules of such a gas per cubic centimeter at 0° C., 76 cm., we have Or if represent the number of molecules per cubic centimeter at 15° C. 76 cm., we should have to multiply the last number by the ratio of absolute temperatures, i.e., by and should obtain then