212 ANXUAL OF SCIENTIFIC DISCOVERY. 



very few physical investigations. No instrument of any kind can be 

 introduced into it, nor even any substance which is acted on by mer- 

 cury. The vacuum obtained by the exhausting pump is not liable to 

 these objections; but even with machines of the most perfect con- 

 struction, and in the best order, a very imperfect approach can be 

 attained to a complete exhaustion. A good ordinary pump with silk 

 valves seldom produces an exhaustion of 0.2 inch ; and it is very rare 

 indeed, if the manometer is properly constructed, to have it carried to 

 0.1 inch. In his " Etudes Hygrometriques," (,4m?. de Chim. 3d Series, 

 vol. xv. p. 190.) M. Regnault has given the following method for push- 

 ing the exhaustion further after the valves have ceased to act. In a 

 large glass globe of from 20 to 25 litres capacity (4^- to 5^- English 

 gallons,) he places an hermetically sealed capsule 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 vapor of water, which has displaced nearly all the residual 

 air in the receiver, 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 open- 

 ing the stop-cocks, the air becomes uniformly diffused through the 

 two spaces ; and if the capacity of the globe is considerable compared 

 with that of the other vessel, the elastic force of the air may be re- 

 duced 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 diffi- 

 culties in the execution, which will at once occur to any one accus- 



* v 



tomed 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 measure- 

 ment, 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, till it must become at last not less com- 

 plete than the Torricellian vacuum ; while at the same time, by sup- 

 pressing the manometer, the existence of mercurial vapor may be 

 altogether prevented. The manipulations required to arrive at this 

 result will not interfere with the presence of the most delicate instru- 

 ments in the reciever. 



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

 exhaust further than to 0-3 inch, or even 0-5 inch, but which must 



