LIQUID AIR. 35 



one another in the least. Each science is fully justified in its own 

 deductions, but must be content to leave the results of others in peace. 

 Such is the ultimate conclusion to which all the latest authority is 

 tending. Only by a careful comparison of data from each sphere of 

 investigation may we finally hope to combine them all in a composite 

 whole, as many-sided and complex as the life and nature of man itself. 



LIQUID AIR. 



BY IRA EEMSEN, 



PROFESSOR OF CHEMISTRY IN THE JOHNS HOPKINS ITNIVSHSITr. 



~YTT~ATER, the substance most familiar to us, is known in the 

 liquid, in the solid, and in the gaseous state. Everybody 

 knows that by heating the solid it passes into the liquid state, and 

 that by heating the liquid it passes into the form of gas or vapor. 

 So also everybody knows that when the vapor of water is cooled it 

 is liquefied, and that by cooling liquid water sufficiently it becomes 

 solid or turns to ice. In the same way many of the substances that are 

 known to us as liquids, such as alcohol and ether, can be converted 

 into the form of gas or vapor by heat. In fact, this is true of most 

 liquids. The temperature at which a solid passes into the liquid 

 state is called its melting point, and the temperature at which a liquid 

 passes into the gaseous state is called its boiling point. The boiling 

 point of water, for example, is 100 C. (212 F.) in the open air. But 

 the boiling point varies with the pressure exerted upon the surface. 

 The pressure that we ordinarily have to deal with is that of the 

 atmosphere. If the pressure is increased the boiling point is raised, 

 and if the pressure is decreased the boiling point is lowered. In 

 dealing, then, with the conversion of a gas into a liquid, or that 

 of a liquid into a gas, both the temperature and the pressure have 

 to be considered. 



Just as water is most familiar to us in the liquid form, so there 

 are substances that are most familiar to us in the gaseous form. 

 In fact, the only gaseous substances that can be said to be familiar 

 to everybody are the gases contained in the air. The principal con- 

 stituents of the air are nitrogen and oxygen, which form respect- 

 ively about four fifths and one fifth of its bulk. Besides these gases, 

 however, the air contains water vapor, carbonic-acid gas, ammonia, 

 argon in small quantities, and many other substances in still smaller 

 quantities. For the purposes of this article it is only necessary to 

 have in mind the nitrogen, oxygen, water vapor, and carbonic acid. 

 Of these, the water vapor is easily converted into liquid, as, for ex- 



