in the Receiver of an Air-pump. 107 



of desiccating completely the potash solution without becoming 

 itself notably diminished in strength, but at the same time does 

 not expose so large a surface as to convert the potash into a dry 

 mass in less than five or six hours at the least. The pump is in 

 the first place worked till the air in the receiver has an elastic 

 force of 03 or 0*4 inch, and the stop-cock below the plate is 

 then closed. A communication is now established between the 

 tube for admitting air below the valves and a gas-holder con- 

 taining carbonic acid, which has been carefully prepared so as 

 to exclude the presence of atmospheric air. After all the air 

 has been completely removed from the connecting tubes by alter- 

 nately exhausting and admitting carbonic acid, the stop-cock 

 below the plate is opened and the carbonic acid allowed to pass 

 into the receiver. The exhaustion is again quickly performed to 

 about the extent of half an inch or less. If a very perfect vacuum 

 is desired, this operation may be again repeated j and if extreme 

 accuracy is required, it may be performed a third time. It is 

 not likely that anything coidd be gained by carrying the process 

 further. On leaving the apparatus to itself, the carbonic acid 

 which has displaced the residual air is absorbed by the alkaline 

 solution, and the aqueous vapour is afterwards removed by the 

 sulphuric acid. The vacuum thus obtained is so perfect, that 

 even after two operations it exercises no appreciable tension. 



To give a clear conception of the progress of the absorption, 

 I will describe in detail one observation in which the tension 

 was measured simultaneously by a good syphon-gauge and by a 

 manometer, formed of a barometric tube - 5 inch in diameter, 

 inverted in the same reservoir of mercury as a similar tube com- 

 municating with the interior of the receiver. The barometer had 

 been carefully filled, and the depression of the mercury estimated 

 by the method already described at less than j^i^dth of an inch. 



Previous to the admission of the carbonic acid, the exhaustion 

 was carried only to - 4 inch ; it was again carried to 1 inch ; 

 and a third time to 0*5 inch, after which the apparatus was left 

 to itself. The manometer indicated a pressure in — 



15' of 0-25 inch. 

 30' ... 0-17 ... 

 80' ... 0-10 ... 

 200' ... 0-02 ... 



In twelve hours the difference of level was just perceptible, when 

 a perfectly level surface was brought down behind the tubes till 

 the light was just excluded. In thirty-six hours not the slight- 

 est difference of level could be detected. The vacuum has re- 

 mained without the slightest change for fourteen days. 



It is evident that the only limit to the completeness of the 



