FUNDAMENTAL PROPERTIES OF MATTER. 35 



Barometer. A second method by which may be demonstrated 

 the fact that atmospheric air possesses weight, is by means of the 

 barometer. The atmosphere is that ocean of gas which encircles the 

 earth with a layer some 50 or 100 miles in thickness, exerting a con- 

 siderable pressure upon all substances by its weight. The instru- 

 ments used for measuring that pressure are known as barometers, and 

 the most common form of these is the mercury barometer. It may 

 be constructed by filling with mercury a glass tube closed at one end 

 (and about three feet long) and then inverting it in a vessel contain- 

 ing mercury, when it will be found that the mercury no longer fills 

 the tube to the top,' but only to a height of about 30 inches, leaving 

 a vacuum above. The column of mercury is maintained at this 

 height by the pressure of the, atmosphere upon the surface of the 

 mercury in the vessel ; a column of mercury about 30 inches high 

 must consequently exert a pressure equal to the pressure of a column 

 of the atmosphere of the same diameter as that of the mercury 

 column. 



As the weight of a column of mercury, having a base of one square 

 inch and a height of about 30 inches, is equal to about 15 pounds, a 

 column of atmosphere having also a base of one square inch must also 

 weigh 15 pounds. In other words, the atmospheric pressure is equal 

 to about 15 pounds to the square inch, or about one ton to the square 

 foot. This enormous pressure is borne without inconvenience by the 

 animal frame in consequence of the perfect uniformity of the pressure 

 in every direction. 



A barometer may be constructed of other liquids than mercury, but as the 

 height of the column must always bear an inverse proportion to the density of 

 the liquid used, the length of the tube required must be greater for lighter 

 liquids. As water is 13.6 times lighter than mercury, the height of a water 

 column to balance the atmospheric pressure is 13.6 times 30 inches, or about 34 

 feet, which would, therefore, be the height of the column of water required. 



It is evident that the pressure of the atmosphere is equal to the weight of a 

 column of mercury, and also that this weight is directly proportional to the 

 length of the mercury column. Hence, different atmospheric pressures can be 

 compared in terms of the length of the mercury column of the barometer, 

 instead of in terms of pounds per square inch. For example, at a pressure of 

 15 pounds per square inch, the mercury column is about 30 inches, while at a 

 pressure of 10 pounds per square inch, it is 20 inches. ^ The ratio of the pres- 

 sures in pounds is 10 : 15 or 2 : 3, and the ratio of the mercury columns is 20 : 30 

 or 2 : 3, which is the same. This fact is of interest in experiments in which 

 gas volumes are dealt with, because calculations have to be made involving a 

 ratio of pressure*, and since this ratio is the same as that of the lengths of the 

 mercury column corresponding to the pressures, it simplifies matters greatly to 

 express pressures in terms of the length of a column of mercury. 



