234 BULLETIN OF THE 



ing steam under great pressure, increasing in a higher ratio than 

 the increased temperature required for the pressure. And finally 

 lie exhibited the striking paradox, that the jet of saturated steam 

 from a boiler will not scald the hand exposed to it, at a prescribed 

 near distance from the try-cock, provided the steam be sufficiently 

 hot.* 



Prolific and skilful in devising experiments, Henry delighted 

 in making evident to the senses the principles he wished to im- 

 press upon the mind. Extending the law of cooling by expansion, 

 from steam at high temperatures, to air at ordinary temperatures, 

 his next communication to the Institute (made March 2nd 1825,) 

 was "On the Production of Cold by the Rarefaction of Air." 

 As before, he accompanied his remarks by several characteristic 

 exhibitions. 



"One of these experiments most strikingly illustrated the great 

 reduction of temperature which takes place on the sudden rare- 

 faction of condensed air. Half a pint of water was poured into 

 a strong copper vessel of a globular form, and having a capacity 

 of five gallons; a tube of one-fourth of an inch caliber with a 

 number of holes near the lower end, and a stop-cock attached to 

 the other extremity, was firmly screwed into the neck of the 

 vessel ; the lower end of the tube dipped into the water, but a 

 number of the holes were above the surface of the liquid, so that 

 a jet of air mingled with water might be thrown from the foun- 

 tain. The apparatus was then charged with condensed air, by 

 means of a powerful condensing pump, until the pressure was 

 estimated at nine atmospheres. During the condensation the 

 vessel became sensibly warm. After suffering the apparatus to 

 cool down to the temperature of the room, the stop-cock was 

 opened: the air rushed out with great violence, carrying with it 

 a quantity of water, which was instantly converted into snow. 

 After a few seconds, the tube became filled with ice, which almost 

 entirely stopped the current of air. The neck of the vessel was 

 then partially unscrewed, so as to allow the condensed air to rush 

 out around the sides of the sci-ew: in this state the temperature 

 of the whole interior atmosphere was so much reduced as to 

 freeze the remaining water in the vessel. '"f 



Although the principle on which this striking result was based 

 was not at that time new, it must be borne in mind that this 



* While it requires a beat of 2.500 F. to generate a steam-prpssnre of two 

 atmospheres ()'. e. one additional to the existing), 25° hieherwill produce 

 a pressure of three atmospheres, and lOOO higher, (or 3550 p.) will produce 

 a pressure of nine atmospheres: the curve (by rectangular co-ordinates 

 of temperature and pressure) resembling a hyperbola. The increased 

 velocity at high pressure produces a molecular momentum of expansion 

 carrying the rarefaction beyond the limit of atmospheric pressure ; and in 

 the case of the exposed hand, the injected air current doubtless adds to 

 the cooling impression. 



t Trans, Albany Inst. vol. i. part 2, p. 33. 



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