278 ATOMIC THEORY. 



precipitation of metals by each other, which led the way 

 to the atomic theory of Dr. Dalton, to whom entirely 

 belongs the observation, that the equivalent of a com- 

 pound body is the sum of the equivalents of its consti- 

 tuents, and the discovery of combination in multiple 

 proportions. 



The elements of a molecule can take a new arrange- 

 ment amongst themselves, without any alteration in the 

 number of the atoms or of their weight, and thus give 

 rise to a body of a different form and colour, although 

 possessing the same chemical constitution. This is the 

 case with many of the organic compounds of carbon and 

 hydrogen. 



The elements of a compound may be disassociated, 

 and thus the dissimilar substances of which it is com- 

 posed set free. A piece of chalk exposed to heat is, by 

 the disturbance of its molecular arrangement, changed 

 in its nature ; a gaseous body, carbonic acid, is liberated, 

 and quick-lime (oxide of calcium) is left behind. If 

 this carbonic acid is passed through red-hot metal tubes, 

 or brought in contact with heated potassium, it is re- 

 solved into oxygen and charcoal the oxygen com- 

 bining with the metal employed. The oxide of calcium 

 (lime), if subjected to the action of a powerful galvanic 

 current, is converted into oxygen and a metal, calcium. 

 Thus we learn that chalk is a body consisting of two 

 compound molecules, carbonic acid, which is formed 

 by the combination of an atom of carbon with two atoms 

 of oxygen, and lime, which results from the union of 

 an atom of calcium with one of oxygen. 



The condition requisite to the production of chemical 

 action between bodies is that they should be dissimilar. 

 Two elementary atoms are placed within the spheres of 

 each other's influences, and a compound molecule 

 results. Oxygen and hydrogen form water; oxygen 

 and carbon give rise to carbonic acid ; nitrogen and 

 hydrogen unite to form ammonia; and chlorine and 



