HISTORY OF STEAM. 275 



on examination to have increased in bulk ; and the same effect takes 

 place in all other bodies, with one or two exceptions, for which no 

 explanation can be given. Now, when water is exposed to the action of 

 heat, it also expands and assumes a gaseous state and this is steam. In 

 such condition it is very elastic, and, like the atmosphere, invisible. 

 This conversion takes place at all temperatures, from the surface of liquids; 

 but the vaporisation, or formation of vapor, is slow, and the vapor 

 produced is prevented from rising by the pressure of the atmosphere, 

 which checks its further formation ; but a current of air, by removing 

 the vapor previously formed, allows the formation of more, which, in its 

 turn is removed, and so the process goes on until all the liquid is 

 evaporated. This explains why, on a windy day, or in a situation 

 exposed to a current of air, vaporisation proceeds so rapidly. 



If, however, the water be heated in an open vessel, till it attains a 

 heat of two hundred and twelve degrees of Farenheit's Thermometer, it 

 is said to have reached the boiling point, and it cannot be made hotter ; 

 all subsequent additions of heat going to form steam, which rises from 

 that part of the vessel nearest the source of heat, and escapes from the 

 surface of the water, producing that agitation commonly called boiling. 

 About one thousand degrees of heat are necessary to convert boiling 

 water into steam; but this accession of heat not being indicated by the 

 Thermometer, is called latent heat. At two hundred and twelve 

 degrees the elastic power of steam is equal to that of the atmosphere, 

 and at four hundred and nineteen degrees, is one thousand and fifty 

 times greater; it then exerts a pressure of nearly fifteen thousand 

 pounds on every square inch of the inside of the vessel in which it may 

 be confined. It has been found by repeated experience, that steam at 

 the temperature of two hundred and ten degrees, occupies about one 

 thousand eight hundred times the space of the water from which it is 

 formed. But till the commencement of the present century this fact 

 was not generally known. It was thought that the expansion was 

 fourteen thousand times greater than the water which produced it; this 

 has, however, since been proved erroneous. It must not be forgotten 

 that steam can be suddenly condensed, however great may be its state of 

 expansion, into the same quantity of water that it formed before the 

 application of heat had changed its condition, and it was for the purpose 

 of making available the force so generated, that many of the first 

 invented steam engines were constructed : it is the expansive force of 

 steam, however, that is the prime mover of the engines of thepresent day. 



