260 EVAPORATION. 



since a plan based on this principle was adopted. In 

 many instances the influence on the human system of 

 vegetable medicines thus obtained is so different from 

 that of the old preparations,, that this principle becomes 

 one of the utmost importance to the medical practitioner. 

 The expansive force of steam is prodigious. At 212 

 degrees, the point at which water boils under common at- 

 mospheric pressure, the force of steam is, as might be 

 supposed, 15 pounds to the square inch, or equal to the 

 pressure of the atmosphere. Above this temperature it 

 increases according to the following table : 



At 250 it is 30 pounds per inch. 

 " 272 " " 45 " " " 

 " 290 " " GO " " " 



Seeing the rapid increase of the expansive force in the 

 above table, we have the explanation of the terrible 

 effects occasionally produced by confined water, when 

 overheated. A boiler of any kind, if completely closed 

 and having no safety valve, will explode as if charged 

 with gunpowder. And against such a disaster, no strength 

 of materials is a sufficient protection without care and 

 skill on the part of those who have the management of 

 this mighty agent ; but with these it may be made, notwith- 

 standing its tremendous power, to perform its part almost 

 if not quite as safe, as the beast of burden that we consider 

 completely under our control. Unhappily the instances 

 are too numerous, in which the incautious or ignorant use 

 of steam has produced explosions, which have shattered 

 buildings, and sometimes destroyed whole neighborhoods. 

 The principle on which the steam engine acts is not dif- 

 ficult to be understood, although it is often supposed to 

 be intelligible to those only who will devote much time to 

 the study of it. He who can understand a pump, can 

 understand a steam engine. It is in fact only a pump in 

 which the fluid is made to impel the piston instead of 

 being impelled by it, that is to say, in which the fluid 

 acts as the power, instead of being the resistance. It 

 may be described simply as a strong barrel or cylinder 

 with a closely fitting piston in it, which is driven up and 

 down by steam admitted alternately above and below from 



