90 WATER STEAM. 



find a cause preventing them, those crystals always 

 assume the same form. The formation of the crys- 

 tals is one motion, and it is a very important one ; 

 because as there is no other substance which crystal- 

 lizes under exactly the same circumstances as com- 

 mon salt, it is by knowing and bringing about that 

 small and invisible motion that we are enabled to 

 give a pure and wholesome relish to our food ; for 

 even common salt, as it exists in nature, is mixed 

 with magnesia and sulphur, and other ingredients 

 which render it unpalatable and unwholesome. 



The previous state of the process is also motion. 

 A certain quantity of water, having a certain degree 

 of heat, is necessary for separating the natural salt 

 into particles so small as to be invisible ; and the 

 quantity that can be dissolved in boiling water is 

 far greater than that which can be dissolved in cold 

 water. Now it is the property of water that it 

 begins to boil at two hundred degrees of the common 

 thermometer, or a little less or more according to 

 the state of the atmosphere. When that is light, 

 boiling water is a little cooler ; and when heavy, it 

 is a little warmer : but the variations are trifling, 

 and not necessary to be taken into account in com- 

 mon observation, though we cannot observe even 

 the operations of nature to proper advantage with- 

 out knowing something of their causes. 



When the water comes to the boiling point, if the 

 surface of it is freely exposed to the air, it never 

 becomes any hotter ; and after it fairly boils, one 

 could not warm it one jot, except it were in a closed 

 vessel, even though it were exposed to a fire of the 

 greatest strength for twelve months. And the water 

 contends for this law of its being, even when it is 

 in small quantity, with strength far exceeding the 

 strength of armies. It is the resistance of water to 

 being heated above the boiling point which has en- 

 abled England to add the steam-engine to the im- 

 plements of her labour ; and thus leave the horses 



