E. PP. Morley — Amount of Moisture in a Gas. 141 



question: How much moisture is left in a gas dried by phos- 

 phorus pentoxide? He proposed to evolve a gas of which we 

 might be certain that it contained no water, to pass it into 

 water, and then to dry it with phosphorus pentoxide. He sug- 

 gested the evolution of dry oxygen by heating fused potassium 

 chlorate. Whether perfectly dry oxygen could be thus obtained 

 remains to be seen ; the task of keeping up a suitable current 

 of oxygen till a few hundred liters shall pass the absorption 

 tubes would involve a good deal of labor. 



Desiring to know the amount of water which sulphuric acid 

 or phosphorus pentoxide fails to remove from a gas, I succeed- 

 ed in devising a method which has made the solution of the 

 problem easy. It permits the determination of the absolute 

 amount of moisture left in a gas by any drying agent ; the 

 maintaining a slow current of air for days or weeks demands 

 attention for only some five minutes each day, so that very 

 large volumes of air may be used, at small velocities, and even 

 if the residual moisture is as little as a hundredth or a thou- 

 sandth of a milligram in a liter, it may be determined with any 

 needed accuracy. 



I devised the method with the intention of applying it first 

 to phosphorus pentoxide. But in the third number of the 

 Zeitschrift fur analytische Chemie for 1884, Mathesius made 

 certain statements about the use of sulphuric acid in drying 

 tubes, in consequence of which I first undertook the study of 

 the absolute amount of moisture left in a "gas by this drying 

 agent. 



The paper of Mathesius raised a preliminary question which 

 had to be answered. He found that certain drying tubes filled 

 with sulphuric acid, of specific gravity l - 34, when used to ab- 

 sorb moisture as in organic analysis, lost weight at the rate of 

 five or more decimilligrams an hour. This statement must be 

 taken as referring to sulphuric acid supposed to be pure; be- 

 cause a statement that impure sulphuric acid contained some 

 volatile impurity would hardly be worth publication ; and also 

 because, in order to lessen the loss of weight in his drying 

 tubes, Mathesius diluted the acid somewhat, probably suppos- 

 ing that the vapor of sulphur trioxide escaped and occasioned 

 the loss of weight. 



It is difficult to believe that either water or sulphur trioxide 

 can be given up by pure sulphuric acid to a current of gas in 

 any such quantity as Mathesius observed. Regnault* deter- 

 mined the tension of the vapor of water given off at 20° 0. by 

 sulphuric acid of the formula S0 3 +2H 2 0. This is -15 milli- 

 ms ter, so that a liter of absolutely dry air passing through such 

 acid would take up at this temperature '16 milligrams of water. 

 * Ann. Chim. Phys., 3d Series, vol. xv, p. 179. 



