Page:Popular Science Monthly Volume 4.djvu/721

Rh We have also further assumed that the elements, in their gaseous states, have specific gravities corresponding to their atomic weights. It is well known that all gases, whether simple or compound, at the same temperature and pressure, and not near to a condensing point or other change of state, contain precisely the same number of molecules in the same volume. Therefore, it necessarily results that the same measures of the different gases should have weights corresponding to the weights of the molecules of which they are composed. Thus the atom of oxygen is sixteen times as heavy as that of hydrogen; therefore a cubic foot of oxygen gas will weigh sixteen times as much as a cubic foot of hydrogen gas. This is found to be experimentally true of all the gases that can be measured and weighed. The apparent but not real exceptions are that in arsenic and phosphorus two atoms of the element unite to form one molecule of the gas, thus making it twice as heavy as it would be, according to the general rule; while, in the case of mercury and cadmium, the atom divides into two in forming their vapors. Hence we are not absolutely sure in regard to the vapor-molecule, and therefore vapor-density, of such elements as carbon, silicon, and calcium, which chemists have not been able to volatilize. But there is every probability, both from analogy and the position in which some of them are found in the photosphere of the sun, that the vapors of nearly all of them correspond strictly to their combining numbers. The following table, therefore, will show the relative positions, in the atmospheric strata, of some of the most important elements, with the weights of their atoms in hydrogen units, their vapor-densities, compared with air, and the solid specific gravities of some of them as compared with water:

It will be noticed from this table that the elements were arranged in positions most suitable for their combination and deposition, both in the geological order, and in the probable order of their