LIQUID AIR. 45 



tical applications. We may look for these in four directions, to 

 each of which a short paragraph may be devoted: 



First, as a cooling agent. Low temperature is marketable. To 

 be sure, the demand for the extremely low temperature that can be 

 produced by liquid air does not exist to-day, but this concentrated 

 low temperature can be diluted to suit conditions. The only question 

 to be answered in this connection is, then, What is the cost of cold 

 produced by liquid air? It is impossible for any one to answer this 

 question at all satisfactorily at present, It can only be said that 

 this is what experimenters are trying to find out. It appears, how- 

 ever, that they are on the way to cheap liquid air, and that as the 

 processes are improved the price will become lower and lower. 



Second, for the construction of motors. There is no doubt that 

 liquid air with its enormous power of expansion can be used as 

 a source of motive power just as compressed air is. In the case of 

 steam it is necessary to heat the water in order to convert it into 

 steam, and to heat the steam to give it the power of expansion. The 

 cost is, in the first instance, that of the fuel. Given a certain amount 

 of heat, and a certain amount of work is obtained. If liquid air is 

 used, the problem is much the same. Engines must be run in order 

 to compress the air which is to be liquefied. Every gallon of liquid 

 air has been produced at the expense of work of some kind. Now, 

 the question arises at once, What proportion of the work that was 

 put in that gallon of liquid air in the course of its production can 

 be got out of it again? It is certain that all of it can not be got out 

 unless all that we have ever learned about such matters goes for 

 nothing. In dealing with the problem of the application of liquid 

 air as a source of motive power we are therefore doubly handicapped. 

 In the first place, we do not know the cost of the liquid when pro- 

 duced on the large scale; and, in the second place, we do not know 

 the probable efficiency of a liquid-air motor. I say " we do not know." 

 Perhaps Mr. Tripler and the others engaged in the experiments on this 

 subject do know approximately. We certainly can not blame them for 

 not telling us all they know at this stage of the work. It is unfortu- 

 nate, however, that such a statement as was recently published in a 

 popular magazine should be allowed to gain currency apparently 

 with the sanction of Mr. Tripler. The statement referred to is to the 

 effect that ten gallons of liquid air have been made by the use of three 

 gallons of liquid air in the engine. If that means that the ten 

 gallons of liquid air are made from air at the ordinary pressure, the 

 statement is in direct conflict with well-established principles. If 

 it means that the ten gallons of liquid air are made from air that has 

 already been partly compressed, we must know how much work has 

 been done before the liquid-air engine began. Leaving out of con- 



