42 ANNUAL OF SCIENTIFIC DISCOVERY. 



beneath it a shallow vessel, which takes up not only the bell-glass, 

 but also a sufliciont quantity of mercury to keep the gas im- 

 prisoned until the arrangements for the experiment are com- 

 pleted. 



The extreme solubility of ammoniacal gas is, therefore, a 

 property of which advantage may be taken for creating a vacuum, 

 exactly as the same object is accomplished by the condensation of 

 steam. As, on the otlior hand, the pressure which it is capable 

 ot exerting at given temperatures is nmch higiier than that which 

 steam afibrds at the same temperatures ; and as, conversely, this 

 gas requires a temperature considerably lower to produce a given 

 pressure than is required by steam, it seems to possess a combi- 

 nation of properties favorable to the production of an economical 

 motive power. 



Ammonia, like several other of the gases called permanent, 

 may be liquelied by cold and pressure. At a temperature of 

 38.0° C, it becomes liquid at the pressure of the atmosphere. At 

 the boiling-point of water it* requires more than CI atmospheres 

 of pressure to reduce it to liquefaction. The same elfect is pro- 

 duced at the freezing-])oint of water by a pressure of 5 atmos- 

 pheres, at 21° C. (70° F.) by a pressure of 9, and at 38° C. (100° 

 F.) by a pressure of 1-4. 



If a refrigerator could be created having a constant tempera- 

 ture of 0° C, or lower, liquid ammonia would furnish a motive 

 power of great energy, without the use of any artilicial heat. 

 The heat necessary to its evaporation might be sui)plit'd by i)lac- 

 inff the vessel containing it in a water-ljath, fed, at least durinir 

 summer, from any natural stream. Such a condenser could not 

 be economically maintained. A condenser at 21° C, however, 

 and an artificial temperature in the boiler of 38° C, wouhl iurnish 

 a differential pressure of 5 atmospheres, with a maximum pres- 

 sure of 14, By carrying the heat as high as 50° C. (122° F.), a 

 differential pressure of 11 atmospheres could be obtained, with an 

 absolute pressure of 20. 



These pressures are too high to be desirable or safe. More- 

 over, condensation is more easil}' effected by solution than by 

 simple refrigeration, and hence, in the ammoniacal gas engines 

 thus far constructed, the motive power has been derived, not from 

 the liquefied gas, but from the aqueous solution. The gas is ex- 

 pelled from the solution by elevation of temperature. At 50° C. 

 (122° F.) the pressure of the liberated gas is equal to that of the 

 atmosphere. At 80° C. (170° F.) it amounts to 5 atmospheres, 

 and at 100° C. (212° F.) to 7^. At lower temperatures the gas is 

 redissolved, and the pressure correspondingly reduced. 



In the ammoniacal engine, therefore, the expulsion and reso- 

 lution of the gas take the place of vaporization and condensation 

 of vapor in the st<'am engine. The manner of operation of the 

 two descriptions of machine is indeed so entirely >imilar, tliat but 

 for the necessity of providing against the loss of the ammonia 

 they might be used interchangeably. The ammonia engine can 

 alwavs be worked as a steam engine, and the steam enijine can 

 be driven by ammonia, provided the ammonia be permitted to 



