RECALCULATION OF THE ATOMIC WEIGHTS. OXYGEN. The ratio between oxygen and hydrogen is the foundation upon which the entire system of atomic weights depends. Hence, the accuracy of its determination has, from the beginning, been recognized as of extreme importance. A trifling error here may become cumulative when repeated through a moderate series of other ratios. Leaving out of account the earliest researches, which have now only a historical value, we find that three methods have been employed for fixing this important constant. First, the synthesis of water, effected by passing hydrogen gas over red hot oxide of copper. Secondly, the exact determination of the relative density of the two gases. Thirdly, by weighing the quantity of water formed upon the direct union of a known volume of hydrogen with oxygen. The first of these methods has been employed in three leading investigations, namely, by Dulong and Berzelius,* by Dumas, and by Erdmann and Marchand. The essential features of the method are in all cases the same. Hydrogen gas is passed over heated oxide of copper, and the water thus formed is collected and weighed. From this weight and the loss of weight which the oxide undergoes, the exact composition of water is readily calculated. Dulong and Berzelius made but three experiments, with the following results for the percentages of oxygen and hydrogen in in water: These figures, rather roughly determined, and by no means exact enough to meet the requirements of modern science, give a mean value of 16.021 for the atomic weight of oxygen. As the weighings were not reduced to a vacuum, this correction was afterwards applied by Clark,* who showed that these syntheses really make O 15.894; or, in Berzelian terms, if O= 100, H = 12.583. = In 1842 Dumas† published his elaborate investigation upon the composition of water. The first point was to get pure hydrogen. This gas, evolved from zinc and sulphuric acid, might contain oxides of nitrogen, sulphur dioxide, hydrosulphuric acid, and arsenic hydride. These impurities were removed in a series of wash bottles; the HS by a solution of lead nitrate, the H,As by silver sulphate, and the others by caustic potash. Finally, the gas was dried by passing through sulphuric acid, or, in some of the experiments, over phosphorus pentoxide. The copper oxide was thoroughly dried, and the bulb containing it was weighed. By a current of dry hydrogen all the air was expelled from the apparatus, and then, for ten or twelve hours, the oxide of copper was heated to dull redness in a constant stream of the gas. The reduced copper was allowed to cool in an atmosphere of hydrogen. The weighings were made with the bulbs exhausted of air. The following table gives the results: Column A contains the symbol of the drying substance. B gives the weight of the bulb and copper oxide. C, the weight of bulb and reduced copper. D, the weight of the vessel used for collecting the water. E, the same, plus the water. F, the weight of oxygen. G, the weight of water formed. H, the crude equivalent of H when 0 = 10,000. I, the equivalent of H, corrected for the air contained in the sulphuric acid employed. This correction is not explained, and seems to be questionable. * Philosophical Magazine, 3d series, 20, 341. |