STEAM. 89 



prevent its forcing the quicksilver before it and crowding 

 into the boiler through the tube. As before stated, when 

 the thermometer reaches 250, it is found that the denser 

 steam will not only balance the atmosphere, but has 

 force enough to lift the mercury thirty inches, which is 

 equivalent to another atmosphere. Steam at 250, and 

 in contact with water, is therefore said to exert a pres- 

 sure of two atmospheres, or thirty pounds to the square 

 inch. At 275 it has a pressure of three atmospheres j 

 and at 294, of four. 



What is said 214. ALL VAPOR HAS ELASTIC FORCE. 



%rcfofva St ors The a PP aratlls J ust described shows the 

 at low tem- pressure of steam at and above 212 degrees. 

 per But vapor of water has elastic force at all 



temperatures. This is best shown by passing a little 

 water up into a Toricellian vacuum, and observing the 

 effect. The water is introduced by blowing it 

 through a glass tube, one end of which is brought 

 under the mouth of the inverted tube. The 

 drop rises and floats on the mercury, and as 

 vapor forms at all temperatures, a portion of 

 it is immediately converted into vapor. At 

 the same time the level of the mercury is de- 

 pressed. It is crowded down in opposition to 

 the pressure of the air outside, by the elastic 

 force of the vapor formed. For the sake of 

 simplicity, the space above the mercury was 

 supposed to be a vacuum, but the effect is the 

 same as if it is filled with air. For, as has 

 been already shown, vapor is produced as well in 

 air as a vacuum, and with the same elastic force. If 

 the top of the tube is warmed, denser vapor is formed 



