326 THE SCIENTIFIC PAPERS OF 



ration in the boiler. The machine was largely used, especially on 

 the Continent ; and, from information he had received, it pro- 

 duced a hundredweight of ice at the expense of about a shilling. 

 An ingenious modification of this machine for small applications 

 on board ship or for household use had been devised, consisting of 

 two vessels connected by a pipe, but hermetically sealed. One of 

 the vessels contained the mother liquid, which was alternately 

 heated and cooled, to drive off the ammonia, and to re-absorb it 

 from the second vessel, which served alternately as condenser and 

 refrigerator for the production of ice. Another method was by 

 the solution of crystalline substances. There were various re- 

 frigerating mixtures, one of the salts so employed being carbonate 

 of ammonia, and another chloride of calcium. When crystals of 

 chloride of calcium were dissolved in water, a considerable reduc- 

 tion of temperature about 30 Fahr. took place. Although that 

 would not be sufficient to produce ice from water of 60 or 65" 

 temperature, an arrangement could be made by which the water 

 to be cooled exchanged heat with the spent liquor, thus producing 

 an accumulation of the effect in the centre of the machine. He 

 constructed a machine on this principle many years ago, which 

 produced ice at a considerable rate, but the salt employed 

 chloride of calcium was not a pleasant substance to deal with. 

 It had to be re-evaporated and crystallised, and this process was 

 inferior to the purely mechanical methods which had since been 

 adopted. The most perfect of these, as regarded cleanliness and 

 freedom from loss, was the air machine. Atmospheric air was 

 compressed to one half or one atmosphere above atmospheric 

 pressure. The compressed air was allowed to cool in contact 

 with water, either by external application or by injection, and to 

 expand again in a working cylinder. The amount of heat that 

 disappeared in the second working cylinder was the exact measure 

 of the refrigeration produced, and it could be easily calculated ; 

 whereas the power expended was the difference of force involved 

 in compressing the air at a higher and of expanding it at a lower 

 temperature. In 1857 a machine of that description was invented 

 by Dr. Gorrie, an American, and was brought to London. 

 Mr. Siemens was asked to report upon it. The machine did not 

 produce satisfactory results. The engine was a good one, and the 

 air-pump was judiciously constructed ; but the connection between 



