THE effusion method of determining gas density, which is based upon the fact that the times required for the escape of equal volumes of two gases under the same pressure through the same small orifice are approximately proportional to the square roots of the densities of the gases, was investigated in order to determine the accuracy of the method and its sources of error. In co6peration with 'a. number of men employing this method in the natural gas industry, a series of experiments was made, using their apparatus under field conditions. It was found that results in error by more than Io per cent. were not unusual.
The theory of the effusion process was studied in order to determine the influence of the numerous variables affecting the apparent specific gravity. The effect of differences in physical properties upon the relative rates of effusion of air and hydrogen, argon, methane, and carbon dioxide at different pressures was determined. A more detailed treatment of the theory in the light of these results will be given in ano.ther paper. The facts observed in this study, together with the observations made on the effect of the effusion pressure, the confining medium, and the sh~ape and size of the orifice, have been used in determining the most favorable conditions of operation for this method.
It is very important that the orifice be of the proper size and shape. Recommendations have been made as to the most suitable type and form of apparatus for use and specifications given to guide in the construction of the orifice. * Communicated by the Director. 1 Technologic Paper No. 94.