ON ARTIFICIAL EEP^RIGERATION. 951 



erating agent here, by abstraction of beat from the body to be cooled, 

 reassumes a gaseous or vaporous form. 



IV. A compressor, or pump, which draws off from the refrigerator the 

 vapor or gas as it forms, and returns it to the boiler or heater. I prefer 

 to use for this i^urpose a double pump, analogous to the double engine, 

 and driven by the engine. The A^apor is returned to the heater or boiler 

 against the pressure of the gas therein, and it is for this pm^iose that I 

 prefer the double pump, in which the compression is eftected by steps 

 or successive stages, and not all at once. The vapor is comj)ressed to a 

 certain extent in the first cyUnder; it thence passes to the second cylin- 

 der, where it is still further compressed and forced back into the boiler. 

 The pump is provided with the usual puj)pet or check valve, for prevent- 

 ing back-pressure ; said valves opening only at the concluding j)art of 

 each revolution or stroke of their respective pistons. 



V. Between the pumps and the boiler, heat may be rejected in a sur- 

 face condenser in which water is the cu-culating medium, and the lique- 

 fied ammonia at the temperature of the surrounding water may then 

 be forced by an injector or by a special force-pump into the ammonia 

 heater. 



VI. By the use of a steam-engine aiding the compression and practi- 

 cally lifting the ammonia directly from the refrigerator to the ammonia 

 heater, no direct heat need be applied to the latter, and the amount 

 equivalent to the energy developed by the steam-engine takes its place. 



Subject strictly to thermodynamic law, a maximum economy can by 

 these means be obtained for the abstraction of heat from surrounding 

 objects. 



It is evident, inasmuch as the heat abstracted from the agent to be 

 cooled is availed of, comparatively little additional heat is requii-ed to 

 brmg the vajjor or gas to the condition for imparting motion to the en- 

 gine piston. 



I propose to distinguish the apparatus in which this cycle of opera- 

 tions can be conducted by the name of " Thermo-glacial Engine." 



T.— DEY COLD AIR AS A PEESEEVATIVE. 



"We learn, from Turner's Embassy to Thibet,* that the flesh of ani- 

 mals is preserved frost-dried — not frozen^and it keeps without salt. 

 He says: "I had supplies of this prepared meat during all the time I 

 remained at Teshoo Loomboo, which had been cured in the preceding 

 winter. It was perfectly sweet, and I was accustomed to eat heartily of 

 it, without any further dressing, and at length grew fond of it. It had 

 not the appearance of being raw, but resembled in color that which has 

 been well boiled. It had been deprived of all ruddiness by the intense 

 cold." 



Erozen meat, unless losing actively in weight by evaporation owing 



* 4°, Loudon, 1806, p. 301. Quoted by Samuel Parker in Ms Chemical Essays. Bohn, 

 1841. London. 



