PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 377 



cloudless night, and the small pellicle which is formed upon the 

 surface from the evaporation and radiation, is carefully gathered 

 and sold. This is very expensive. 



Ice has been produced by dissolving various salts (the nitrate 

 of ammonia, for example) in water, or in acids. After examination 

 of this method by a French society similar to ours, it was decided 

 that the ice produced in that way would cost over five cents 

 per pound. Dr. Gorrey proposed to condense air, to squeeze the 

 heat out of it, so to speak, and to allow it to expand in contact 

 with Avater, which it would freeze. With his ingeniously con- 

 structed apparatus, he has produced five or six thousand pounds 

 of ice in a day ; but at a cost a good deal higher than the natural 

 ice — a cost of forty dollars per ton instead of three dollars. 

 Prof Twining has a plan for making ice by the evaporation of 

 ether. He has built several machines ; and, I presume there is 

 no doubt they are better than any others. 



The theoretical cost of the manufacture of ice is very easily 

 calculated. There is a relation between all the physical forces 

 by which one may be measured by another. A certain amount 

 of electricity is worth a certain amount of heat. So many foot- 

 pounds are equal to so much heat, or so much electricity, or so 

 much cold ; for the same amount of power which would be 

 required to increase the temperature of a body by ten degrees, 

 would be required to diminish it by ten degrees. 



It has been proposed to generate heat by mechanical means. 

 Water has been boiled by the heat produced by friction. 

 Water may also be frozen by mechanical force. Knowing how 

 much heat a given power will produce, we know how much cold 

 it will produce, and consequently how much ice. 



The apparatus of Mr. Gorrey or of Prof Twining, if applied 

 merely to cool air ten or twenty degrees, which would be all 

 that would be requisite, would probably be of more value than 

 applied to the manufacture of ice, because there need be no 

 loss by radiation. 



Mr. Fisher. — A pound of coal will melt one hundred pounds 

 of ice. Or it will melt sixty pounds and raise it to the tempera- 

 ture of fifty or sixty degrees, when it will be no longer useful 

 for cooling. So that a ton of coal will do as much as sixty tons 

 of ice. I apprehend that if we used ice for cooling the air, we 

 could get it much cheaper than we now do ; for that which we 

 now use is derived from the best of water, and brought a long 



