378 Prof. Hare's Apparatus for Freezing Water. 



process, in order to obviate the causes of disappointment. It 

 appeared to me that the failure arose from imperfection in 

 the vacuum. An excellent pump, with perfectly airtight 

 cocks, is indispensable; and not only must the pump be well 

 made, it must likewise be in good order. Neither should the 

 packing of the pistions, the valves, nor the cocks, allow the 

 slightest leakage. If a pump has been used previously for 

 freezing by the evaporization of aether, it will not be compe- 

 tent for the experiment in question, unless it be taken apart 

 and cleaned. 



Cocks of the ordinary construction are rarely if ever per- 

 fectly airtight, and their imperfection always increases with 

 wear. Under these impressions, having cleansed my air- 

 pump, and put it into the best order possible ; for the purpose 

 of obviating leakage through the cocks associated with the 

 instrument, I closed the hole in the centre of the air-pump 

 plate by a screw, and for a receiver made use of a bell glass 

 with a perforated neck, furnished with a brass cap and a fe- 

 male screw, by means of which one of my valve cocks was 

 attached. A communication between the bell and the cham- 

 bers of my pump was established through the valve cock and 

 a flexible lead pipe, in a mode analogous to that already de- 

 scribed in the account of the valve cock. In this way I suc- 

 ceeded in preserving the vacuum longer than when the cocks 

 of the air-pump were employed in the process, and accom- 

 plished the congelation of water by means of the vacuum and 

 sulphuric acid. 



Latterly, I have used an apparatus in which a brass cover 

 is made to close a large glass jar so as to be quite tight. In ope- 

 rating, the bottom of the jar was covered with sulphuric acid, 

 and another jar with feet, also supplied with acid enough to 

 make a stratum half an inch deep on the bottom, was intro- 

 duced as represented. The bottom of the vessel last men* 

 tioned was, by means of the feet, kept at such a height above 

 the surface of the acid in the outer jar as not to touch it. 

 Upon the surface of the glass vessel, a small piece of very thin 

 sheet brass was placed, made concave in the middle, so as to 

 hold a small quantity of water. 



The brass cover was furnished with three valve cocks, one 

 communicating with the air-pump, another with a barometer 

 gauge, and the third with a funnel supplied with water. 

 Under these circumstances, having made a vacuum on a Sa- 

 turday, I was enabled to freeze water situated on the brass, 

 and to keep up the congelation till the Thursday following. 

 As the water in the state of ice evaporates probably as fast 

 as when liquid, during the night the whole quantity frozen 



