106 Dr. Andrews on a Method of obtaining a perfect Vacuum 



(4i to 5i English gallons), lie places an hermetically sealed cap- 

 sule of glass containing from 40 to 50 grms. of sulphuric acid. 

 He also introduces into the globe 2 or 3 grms. of water, and 

 exhausts till the water has entirely disappeared and the machine 

 ceases to act. By agitating the globe, the capsule is ruptured ; 

 when the sulphuric acid coming into contact with the vapour of 

 water, which has displaced nearly all the residual air in the re- 

 ceiver, condenses it and leaves a vacuum nearly perfect. This 

 globe thus exhausted is next placed in communication with the 

 apparatus in which a very perfect vacuum is desired, taking care 

 to remove the air from the interior of the connecting tubes. On 

 opening the stop-cocks, the air becomes uniformly diffused 

 through the two spaces ; and if the capacity of the globe is consi- 

 derable compared with that of the other vessel, the elastic force 

 of the air may be reduced to a small fraction of a millimetre. If, 

 on the contrary, the capacity of the latter is considerable, this 

 operation must be repeated several times. 



This ingenious process is not adapted to give a very perfect 

 vacuum in the second vessel, unless the operation be repeated 

 several times, which would be exceedingly laborious. It is also 

 liable to other difficulties in the execution, which will at once 

 occur to any one accustomed to experiments of this kind. 

 Besides, it does not afford the means of obtaining a vacuum, 

 which, as far as the indications of a mercurial manometer can be 

 observed, is perfect ; as in M. Regnault's observations, the elastic 

 force of the air was still capable of measurement, although only 

 amounting to a small fraction of a millimetre. 



By using the necessary precautions, a vacuum may be obtained 

 by the following process, with very little trouble, in the ordinary 

 receiver of an air-pump, so perfect that the residual air exerts no 

 appreciable elastic force. Even after this limit has been reached, 

 the exhaustion may be pushed still further, tdl it must become 

 at last not less complete than the Torricellian vacuum ; while at 

 the same time, by suppressing the manometer, the existence of 

 mercurial vapour may be altogether prevented. The manipula- 

 tions required to arrive at this result will not interfere with the 

 presence of the most delicate instruments in the receiver. 



Into the receiver of an ordinary air-pump, which is not re- 

 quired to exhaust further than to 03 inch, or even 0*5 inch, 

 but which must retain the exhaustion perfectly for any length 

 of time, two open vessels are introduced, one of which may be 

 conveniently placed above the other ; the lower vessel containing 

 concentrated sulphuric acid, the upper a thin layer of a solution 

 of caustic potash, which has been recently concentrated by ebul- 

 lition. The precise quantities of these liquids is not a matter of 

 importance, provided they are so adjusted that the acid is capable 



