192 NINTH ANNUAL REPORT OF 



METEOROLOGY. 



Second Lecture. 



That water placed in an open vessel over a fire does not have its 

 temperature raised above 212 degrees, however great the heat may be, 

 and that tlie steam produced is no warmer, is among the first of the 

 wonders which arrests the attention of the student entering upon the 

 field of physical science. The heat of the fire is absorbed in the pro- 

 duction of the steam. .The atoms of water are made to repel each 

 other. A cubic inch of this liquid converted into steam at 212 degrees, 

 at the ordinary pressure of the atmosphere, has its bulk increased 

 nearly 1,700 times. The elasticity of the steam thus produced is 

 equal to the weight of a perpendicular column of 30 inches of mercury. 

 This must appear quite evident when we consider how steam is formed. 

 It rises from the bottom of the vessel whicli contains the water, in 

 bubbles, and can only be rapidly formed when the repulsion of the 

 atoms is able to resist the whole weight of the incumbent atmosphere. 



Dalton, however, discovered that vapor is formed at all tempera- 

 tures ; that the boiling point of water and the elasticity of the vapor 

 are entirely regulated by heat and pressure. This he proved by allow- 

 ing a small quantity of" water to ascend to the top of the mercury in a 

 common barometer tube, where it produced vapor, the elasticity or 

 pressure of which could be exactly measured by the height of the 

 column, as the heat was increased or diminished. This simple experi- 

 ment showed that the connexion between the temperature, density, and 

 elasticity of steam is subject to the same physical law which regulates 

 the density and elasticity of the permanent gases. The following 

 exhibits a few cases of steam at diffisrent temperatures, with the corre- 

 sponding force and weight : 



Temp. 0°; force 0.068 in inches; weight of cubic foot, 0.856 grains. 



" " 1.688 " " 



3.239 

 " " 6.222 " 



11.333 

 257.218 



In fact, after allowing for the difference of the temperature in steam, 

 according to the law wliich holds in reference to other gases, the pres- 

 sure of steam at any low temperature being given, the weight of a 

 cubic foot of steam can be calculated by the simple rule of propor- 

 tion. 



