K W. Morley — Amount of Moisture in a Gas. 143 



This being determined, the way was clear to determine the 

 absolute amount of moisture left in a gas by sulphuric acid. To 

 Liebig's potash bulbs I fused a sixth bulb, connected with 

 the others by a capillary tube of so small bore that when a 

 vacuum was maintained at one end, one or two cubic centi- 

 meters of air would pass through it in a minute. In this sixth 

 bulb I placed sulphuric acid so diluted with water that air pass- 

 ing through it would take up a certain small amount of water. 

 In the bulbs which belonged to the original apparatus was 

 placed sulphuric acid* of specific gravity 1-8381 at 18° C, 

 compared with water at 4° C., weights and thermometer being 

 corrected. 



In use, a partial vacuum was maintained in the five bulbs 

 containing strong acid, while the dilute acid was in contact with 

 air at ordinary pressure. Air in passing from the dilute acid 

 through the constriction towards the strong acid would there- 

 fore expand a number of times depending on the pressure in 

 the partial vacuum. The air before entering the dilute acid 

 was made as dry as sulphuric acid can render a gas; it took up 

 water from the dilute acid ; it was expanded ; the increased 

 volume was made as dry as sulphuric acid can render a gas. 



Let us imagine, for the sake of clearness, that the expansion 

 in passing the constriction was ten times, that five liters of air 

 entered the dilute acid, and that therefore fifty liters passed out 

 of the strong acid. Let us also make two alternative supposi- 

 tions in order; first, that sulphuric acid makes a gas perfectly 

 dry, and second, that it leaves a hundredth part of a milligram 

 of water in a liter of gas. 



If, according to the first supposition, sulphuric acid makes a 

 gas perfectly dry, the five liters of dry air which enter the six- 

 bulb apparatus carry into it no water. In the sixth bulb they 

 take up a small quantity of water. Passing the constriction, 

 they become fifty liters. The sulphuric acid makes the fifty 

 liters perfectly dry, and no water is carried out of the apparatus. 

 Therefore the only effect changing the weight of the apparatus 

 is the escape of sulphur trioxide, the amount of which is ap- 

 proximately known. 



But, secondly, if sulphuric acid leaves a hundredth of a 

 milligram of water in a liter of gas, the five liters of air enter- 

 ing the apparatus carry into it one-twentieth of a milligram of 

 water. In the sixth bulb, more water is taken up. The five 

 liters expand to fifty. Now the drying such a gas as air is 

 simply a process of reducing the vapor tension of the accom- 

 panying vapor of water. One liter of air dried by sulphuric 

 acid will contain water possessing a certain tension, whatever 

 be the pressure of the air. At least Regnault proved this to be 

 true within one per cent. The fifty liters of air will therefore 



