ON ARTIFICIAL REFRIGERATION. 907 



same fuel will give 1 pound of ice per pomid of coal burned if air is 

 being compressed, 3 pounds per pound if ether is used, 5 j)Ounds "with 

 sulphurous oxide, and at least from 10 to 15 with anhydrous ammonia. 

 The efficiency of the pump, the varying amount of heat of compression 

 in pumping different gases in relation to the heat transferred from the 

 refrigerator to the condenser, and lastly the temperature of available 

 condensing water tend to produce the most complex conditions which a 

 skilful observer can well be called upon to study. 



E.— TYPES OF ICE-MACHINES. 



Uj) to the present time there have been five distinct types of ice- 

 machines in the market. 



Firstly. The domestic machines, in which salts are liquefied, as ex- 

 plained elsewhere. Ash's i)iston freezer and Toselh's frigorific mixture 

 machines belong to this group. 



Secondlj". The form of apparatus, in which, in addition to the power 

 used, usually by hand, in working an air-pump to favor the evapora- 

 tion of water to be frozen, we have to calculate the cost of absorbing 

 watery vapor by sulphuric acid in vacuo. 



Thirdly. Distillation or absorption machines, in which heat has to sep- 

 arate a chemical, say ammonia, from water, and in which the cooling 

 water has to favor the reabsorption of the ammonia gas heated in the 

 refrigerator. 



Fourthly. An air-pump with a so-called regenerator or api^liances 

 (whether a second j)ump or otherwise) to cause work to be done in ex- 

 pansion by the compressed air. This is based on Joule's law, subject 

 to slight deviation, that to effect change of temperature air must he allowed 

 to expand in such a manner as to develop mechanical poicer. 



Fifthly. The machines composed of a refrigerator and conde'nser, with 

 an intervening exhausting and condensing pump driven by an engine. 



F.— THEEMODYNAMIC LAWS. 



It is heat we use as the great agent for producing those changes in 

 the physical state of matter whereby the conversion from a gas to 

 liquid, and from a liquid back to gas is effected. There is a definite 

 relation between heat and work as enunciated in the First Law of thermo- 

 dynamics.* Seat and mechanical energy are mutually convertible, a unit 

 of heat corresponding to a certain fixed amount ofworl; called the mechan- 

 ical equivalent of heat. Joule has experimented on the heat x)roduced by 

 the agitation of water, and his latest determinations have proved the 

 correctness of the late Professor Eankine's calculations, viz, that the rais- 

 ing of 1 pound of water 1 degree Fahr., from 39° to 40°, if whoUy 

 converted into work, will raise a pound weight through 774.1 feet, sub- 



* I -would counsel all persons interested in steam-engines, ice-macliines, &c., to read 

 Tlie Steam Engine considered as a Heat Engine, by James H. Cotterill. London and 

 Kew York, Spon, 1878. 



