254 LAW OF THE EXPANSION OF STSAM. 



tubes than to telegraph by electricity. The tubes are laid 

 under ground, with easy curves; the messages are made 

 into a roll and placed in a light felt carrier, the resistance of 

 which in the tubes in London is only f oz. A current of 

 air, forced into the tube or drawn through it, propels the 

 carrier. In most systems the current of air is steady and 

 continuous, and the carriers are introduced or removed 

 without materially altering the flow of air. 



135. The Law of the Expansion of Steam. When 

 steam is produced in a close vessel, as in the boiler of a 

 steam engine, the density of the steam increases with the 

 temperature ; but so long as the temperature remains the 

 same, the quantity of steam that can be raised from the 

 water is limited, and the steam is generated at its maximum 

 density and pressure for the temperature, whatever this 

 may be; if the temperature falls, a portion of the steam 

 resumes the liquid form, and the density of the steam is 

 diminished. When the steam is in its condition of maxi- 

 mum density, it is said to be saturated, being incapable of 

 vaporizing or absorbing more water into its substance, or 

 increasing its pressure, so long as the temperature re- 

 mains the same. Also, on the contrary, steam will not 

 be generated with less than the maximum quantity of 

 water which it is capable of appropriating from the liquid 

 out of which it ascends. Any change in either one of the 

 three elements of pressure, density, or temperature of steam 

 is necessarily accompanied by a change of the other two. 

 The same density is invariably accompanied by the same 

 pressure and temperature. 



If the volume of steam over water be increased, while the 

 temperature remains constant, then, as long as there is liquid 

 in excess to supply fresh vapor to occupy the increased 

 space, the density will not be diminished, but will remain 

 constant with the pressure. If the source of heat be re- 

 moved, when all the liquid is evaporated, the pressure and 



