A HISTORY OF SCIENCE 



theory of matter in mind, makes it clear that this is 

 precisely what one can never hope to accomplish. 

 Action and reaction are equal and in opposite direc- 

 tions at all stages of the manipulation, and hence, 

 under the most ideal conditions, we must expect to 

 waste as much work in condensing a gas (in actual 

 practice more) as the condensed substance can do in 

 expanding to the original volume. Those enthusiasts 

 who have thought otherwise, and who have been on 

 the point of perfecting an apparatus which will readily 

 and cheaply produce liquid air after the first portion is 

 produced, are really but following the old perpetual- 

 motion-machine will-o'-the-wisp. 



It does not at all follow from this, however, that the 

 energies of liquefied air may not be utilized with enor- 

 mous advantage. It is not always the cheapest form 

 of power- transformer that is the best for all purposes, 

 as the use of the electrical storage battery shows. And 

 so it is quite within the possibilities that a multitude 

 of uses may be found for the employment of liquid air 

 as a motive power, in which its condensed form, its 

 transportability or other properties will give it prece- 

 dence over steam or electricity. It has been suggested, 

 for example, that liquefied gas would seem to afford 

 the motive power par excellence for the flying-machine, 

 once that elusive vehicle is well in harness, since one 

 of the greatest problems here is to reduce the weight 

 of the motor apparatus. In a less degree the same 

 problem enters into the calculations of ships, particu- 

 larly ships of war ; and with them also it may come to 

 pass that a store of liquid air (or other gas) may come 

 to take the place of a far heavier store of coal. It is 



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