314 



EBULLITION. 



pure vapor will be thus cut off from all communication with the air. The in- 

 ferior temperature of the surrounding air, taking heat constantly from the bulb 

 and tube, will deprive the vapor contained in them of the quantity of heat ne- 

 cessary to sustain it in the elastic form, and it will be condensed. The great 

 diminution of bulk which it will suffer will cause a partial vacuum to be pro- 

 duced in the bulb and tube, and the pressure of the atmosphere, acting on the 

 surface of the water in the vessel in which the tube is immersed, will force 

 the water up the tube, and and it will completely fill the bulb. 



That form of the steam-engine called the low-pressure engine, derives its 

 principal mechanical efficacy from this property, by which steam is instrumen- 

 tal in the formation of a vacuum. The moving power in that machine is ren- 

 dered operative by a piston placed in a cylinder, in which it moves steam- 

 tight. The atmospheric air and other gases are expelled from the cylinder and 

 tubes which communicate between it and the boiler by steam, in the same 

 manner exactly as in the experiment just described. Steam is allowed to pass 

 freely from the boiler through the tubes and cylinder, and makes its escape 

 finally through a valve or cock provided for that purpose, un il at length all the 

 atmospheric air is blown from the machine. The cock is then closed, and 

 pure steam only fills every part of the engine. A chamber, called a condenser, 

 which is maintained at a low temperature, by being immersed in cold water, 

 is made to communicate with both ends of the cylinder by means of proper 

 tubes and valves. When the piston is required to descend, the communica- 

 tion between this chamber and the bottom of the cylinder is opened, while a 

 communication is at the same time opened between the boiler and the top of 

 the cylinder. The steam which fills the cylinder below the piston rushes 

 toward the condenser by its elastic force, and is there immediately converted 

 into water by the cold medium with which it is surrounded. The cylinder 

 below the piston, therefore, remains a vacuum ; meanwhile the steam, rushing 

 from the boiler above the piston, forces it downward, until it reaches the bot- 

 tom of the cylinder. The communication between the boiler and the top of 

 the cylinder is now closed, and a communication opened between the boiler 

 and the bottom of the cylinder, and at the same time the communication be- 

 tween the condenser and the bottom of the cylinder is closed, and a commu- 

 nication is opened between the condenser and the top of the cylinder. Under 

 these circumstances, the steam which is above the piston rushes by its elastic 

 force toward the condenser, where it is condensed, and the cylinder above the 

 piston remains a vacuum. Meanwhile the steam from the boiler, rushing into 

 the cylinder below the piston, forces it upward, and the piston ascends to the 

 top of the cylinder ; and in the same way the alternate motion of the piston 

 upward and downward in the cylinder is continued. 



The results of experimental inquiry, as we have seen, justify us in assuming 

 as a universal law, that by the application of a sufficient quantity of heat all 

 solids may be converted into liquids ; and, by the abstraction of a correspond- 

 ing quantity of heat, all liquids may be converted into solids. We have like- 

 wise seen, that, by the supply of heat in sufficient quantities, all liquids may 

 be converted into the vaporous or gaseous form ; and analogy would lead us to 

 infer, that, by the due abstraction of heat, the bodies that exist in the gaseous 

 form might be reduced to liquids. The practical results here, however, fall 

 far short of the anticipations to which analogy leads us. There is a numerous 

 class of bodies existing in the gaseous form, among which atmospheric air may 

 be mentioned as the most obvious, which no means hitherto known have con- 

 verted into liquids. Arguments, however, similar to those which led us to in- 

 fer that charcoal and alcohol are not real exceptions to the liquefaction of 

 solids, and the solidification of liquids, but that they transcend the power of 



