100 ANNUAL OF SCIENTIFIC DISCOVERY. 



in detail and cost of construction, they employ the same general 

 principle in the conversion of water into ice. 



To the general reader it may be well to state here that the fun- 

 damental principle upon which machines of this kind operate is the 

 absorption of heat from surrounding bodies by an expanding 

 substance, the conveying of this heat to some other absorbing 

 body, into which the heat is caused to radiate by the condensation 

 of the conve}'ing substance by mechanical compression, the pass- 

 ing back of the conveying body to extract another modicum of 

 heat from the body to be cooled or frozen, and so on till the 

 desired degree of refrigeration is reached. 



It is a physical law with which perhaps some of our readers 

 are not familiar, that the capacity of any substance for heat that 

 is, its power to absorb heat, and hold it in the latent or insensible 

 state increases with its expansion and decreases with its con- 

 densation. A substance which at ordinary temperatures is a 

 permanent gas will, when compressed, become sensibly heated ; 

 the latent heat which it holds, under ordinary circumstances, being 

 rendered sensible by condensation. If while in this state the 

 sensible heat be taken up by some other substance and conve3 T ed 

 away, the gas in expanding will seize the heat from surrounding 

 bodies, thus reducing their temperature. The gas, on being 

 again compressed, will yield this heat to any substance having a 

 lower temperature. The proportion of heat absorbed during 

 expansion, and emitted under pressure, increases with the degree 

 of alternate condensation and expansion. 



Ammonia, which is a gas of ordinary temperatures, becomes a 

 liquid under a pressure of from 9 to 13 atmospheres, according 

 to the temperature of the surrounding air, emitting a large 

 amount of heat in so doing, which amount must be restored to it 

 before it can expand to its original volume. On this account it is 

 admirably fitted for use in refrigerating apparatus. 



The Tellier machine, besides differing much from the Carre 

 machine in matters of detail, differs from it in its action, the con- 

 densation of the ammonia being in the latter effected entirely by 

 mechanical compression, while in the former the strong affinity 

 of ammonia for water is used in the collection of the gas, the 

 latter being separated from the water again by distillation. 



In the Tellier machine the liquefied ammonia is first received 

 into a strong cylinder, for convenience of transportation. This 

 cylinder being attached to suitable pipes connected with the 

 machine, the opening of certain cocks allows the ammonia to 

 escape into a distributer or a cylinder connected by pipes with the 

 congealer. The congealer is a square box divided into compart- 

 ments by hollow metallic partitions, the compartments being tilled 

 with the water to be frozen, or they may be filled with a solution 

 of chloride of calcium or salt water, in which are placed metallic 

 moulds containing the water to be frozen. The latter is most con- 

 venient when very large cakes are desired. 



The ammonia, passing from the distributer into the hollow metal- 

 lic partitions of the congealer, expands into a gas, absorbing in its 

 expansion a large amount of heat from the fluid contained in the 



