144: E. TT. Morley — Amount of Moisture in a Gas. 



carry out ten times as much moisture as the five liters brought 

 in. Neglecting for a moment the evaporation of sulphur trioxide, 

 the apparatus will lose nine twentieths of a milligram in weight. 

 Conversely, if we knew the expansion to be ten times, and the 

 entering air to be five liters, and the loss of weight to be '45 

 milligram, we could compute the water remaining in each liter 

 to be O'Ol milligram. 



In applying this principle, it was necessary to avoid errors 

 due to leakage of moist air into the apparatus during the long 

 time through which an experiment lasted, and to provide for 

 weighing the six-bulb apparatus so accurately as to make cer- 

 tain the detection of a total effect of a few tenths of a milli- 

 gram. I secured sufficient accuracy in weighing by using as a 

 counterpoise an apparatus of the same shape and same kind of 

 glass, filled with the same acid, and interposed in the same cur- 

 rent of air. To these I fitted ground glass caps as accurately 

 as I could, so that I could leave the two apparatus on the 

 balance for several days without any change in their relative 

 weights; of course after applying corrections for the state of the 

 barometer and thermometer. 



I also fitted to the two apparatus just named, a third, which 

 prevented the diffusion of moist air backward from the 

 vacuum, and a fourth which dried the air before it entered the 

 first and second apparatus, all, to each other, by glass tubes 

 with joints carefully ground which were made tight with a fat 

 from which all matter volatile at common temperatures had 

 been removed. In this way, I could leave the apparatus for 

 weeks with the certainty that moist air could not enter the ap- 

 paratus. 



The measure of the volume of air which enters the apparatus 

 and of the expanded volume which leaves it is easy. The 

 third drying tube which prevents the diffusion of moist air 

 backward was fused, together with a barometer gauge, to a 

 tube leading to an air-tight reservoir of 54*1 liters capacity. 

 When all the drying tubes were in place, the pressure in the 

 reservoir was reduced to such a fraction of an atmosphere that 

 the air passed through the last five bulbs of the six-bulb ap- 

 paratus at the rate of about two liters an hour. This pressure 

 was observed on the barometer gauge. Call the pressure a. 

 On the next day, or sometimes in twelve hours, the pressure 

 was again observed. Call the second pressure b. The pressure 

 in the reservoir was then again reduced, and the apparatus was 

 ready for another day of action. 



Now, disregarding variations of temperature and supposing 

 the barometer constant, remembering that the expansion of the 

 air takes place so slowly that no cooling effect is sensible, we 

 can easily compute the volume of rarefied air which has passed 



