166 NON-METALS AND THEIR COMBINATIONS. 



dry, as 0. The instruments used to determine humidity are called 

 hygrometers. 



An analysis of air maybe made by the following method : A graduated glass 

 tube, containing a measured volume of air, is placed with the open end down- 

 ward into a dish containing mercury. A small piece of phosphorus is then 

 introduced and allowed to remain in contact with the air for several hours, 

 when it gradually combines with the oxygen. The remaining volume of air is 

 chiefly nitrogen, the loss in volume represents oxygen. 



For the determination of carbon dioxide and water, a measured volume of 

 .air is passed through two U-shaped glass tubes. One of these tubes has previ- 

 ously been filled with pieces of calcium chloride, the other tube with pieces of 

 potassium hydroxide, and both tubes have been weighed separately. In pass- 

 ing the measured air through these tubes the first one will retain all the 

 moisture, the second one all the carbon dioxide ; the increase in weight of the 

 tubes at the end of the operation will give the amounts of the two constituents. 



That oxygen is found in the atmosphere in a free state is explained 

 by the fact that all elements having affinity for oxygen have entered 

 into combination with it, while the excess is left uncombined. Mtro- 

 gen is found uncombined, because it has so little affinity for other 

 elements. 



Liquefaction of air on a large scale has been made possible by a process 

 which depends on first subjecting air to a pressure of 2000 pounds to the square 

 inch and then permitting the compressed gases to escape from a needle-point 

 orifice. During the expansion of the gas heat is absorbed, i. e., the air as well 

 as the tubes in which it is contained are cooled off'. The cold thus produced 

 is used to cool another portion of compressed air, which, on expanding, becomes 

 colder than the first portion. By repeating the operation a third time the 

 temperature is brought down to 191 C. and below, and at this temperature 

 liquefaction takes place. 



Liquefied air is a mobile, slightly bluish liquid which can be kept for some 

 little time in open vessels i. e., so long as the temperature of nearly 200 C. 

 below freezing is maintained by the evaporation of the liquid. 



As nitrogen is somewhat more volatile than oxygen, the liquefied air, when 

 permitted to stand in open vessels, becomes gradually richer in oxygen, so that 

 finally a liquid is left containing over 80 per cent, of oxygen. Notwithstand- 

 ing the low temperature of this liquid it acts most energetically as a supporter 

 of combustion. 



Of interest are the changes which are brought about in the physical proper- 

 ties of different bodies when cooled down to nearly 200 by immersion in 

 liquid air. Many malleable metals, many soft or elastic bodies, such as rubber 

 and paraffin, when subjected to this low temperature, become as brittle as badly 

 cooled glass ; changes in color, as well as in other properties, take place also. 



Argon, mentioned above as a normal constituent of air, is a gaseous element, 

 discovered in 1894. It may appear strange that a normal constituent of air, 

 present to the extent of nearly 1 per cent., should have been overlooked for so 

 many years, although air had been carefully analyzed many hundred times. 



