from which C = 0.1165, and this number, according to the conditions of the formula, is to be subtracted from the observed reading. The true reading in the case given is, therefore, 29.298 inches or 744.2 millimeters. The observed reading 747.1 “ ----- And the correction 2.9 “ Unless extremely accurate work be required the correction for temperature is of very little importance in nitrogen determinations in fertilizers. Each instrument sent out by the Weather Bureau is accompanied by a special card of corrections therefor, but these are of small importance in fertilizer work. In order then to get the correct weight of the gas from its volume the reading of the thermometer and barometer at the time of measurement must be carefully noted. However, after the end of the combustion, the azotometer should be carried into another room which has not been affected by the combustion and allowed to stand until it has reached the room temperature. Every true gas changes its volume under varying temperatures at the same rate and this rate is the coefficient of gaseous expansion. For one degree of temperature it amounts to 0.003665 of its volume. Representing the coefficient of expansion by K the volume of the gas as read by V, the volume desired at any temperature by V′, the temperature at which the volume is read by _t_ and the desired temperature by _t′_, the change in volume may be calculated by the following formula: V′ = V[1 + K(_t′_ - _t_)]. _Example._—Let the volume of nitrogen obtained by combustion be thirty-five cubic centimeters, and the temperature of observation 22°. What would be the volume of the gas at 0°? Making the proper substitutions in the formula the equation is reduced to the form below: V′ = 35[1 + 0.003665(0°-22°)] or V′ = 35(1 - 0.08063) = 32.18. Thirty-five cubic centimeters of nitrogen therefore measured at 22° become 32.18 cubic centimeters when measured at 0°. When gases are to be converted into weight after having been determined by volume, their volume at 0° must first be determined; but this volume must also be calculated to some definite barometric pressure. By common consent this pressure has been taken as that exerted by a column of mercury 760 millimeters in height. Since the volume of a gas is inversely proportional to the pressure to which it is subjected, the calculation is made according to that simple formula. Let the reading of the barometer, at the time of taking the volume of gas, be H, and any other pressure desired H′. Then we have the general formula: HV V : V′ = H′ : H; and V′ = ----. H′