DECOMPOSITION OF COMPOUNDS. 57 



produced by the action of one salt solution upon another salt solution, 

 the first solution containing a metal which with the acid of the second 

 solution may form an insoluble compound, which is then invariably 

 produced as a precipitate. For instance : Calcium carbonate, CaCO 3 , 

 is insoluble ; if we bring together two solutions containing a soluble 

 calcium salt and a soluble carbonate, such as calcium chloride, CaCl 2r 

 and sodium carbonate, Na 2 CO 3 , calcium carbonate is precipitated. 



A second general rule may be stated thus : When two substances 

 capable of forming a volatile product are brought together, the reaction 

 generally takes place. As instances may be mentioned the liberation 

 of carbon dioxide from any carbonate by the action of an acid, and 

 the liberation of ammonia gas from ammonium compounds by 

 calcium hydroxide. 



The nascent state. This expression is used of elements at the 

 moment when their atoms leave molecules and have not yet had time 

 to reenter into combination. When in this state the atoms have a 

 much greater energy to combine than after having entered into a 

 combination with other atoms of either the same kind (to form 

 elementary molecules) or of another kind (to form compound mole- 

 cules). White arsenic, As 2 O 3 , is a compound of the metal arsenic 

 with oxygen ; if through a solution of this compound hydrogen gas 

 be allowed to pass, no chemical change takes place. If, however, 

 hydrogen be generated or set free in a solution of white arsenic, then 

 the hydrogen atoms, while in the nascent state, have sufficient energy 

 to combine with both the elements arsenic and oxygen, forming 

 arsenetted hydrogen or arsin, AsH 3 , and water, H 2 O. 



Chemical reaction in its broader sense refers to any chemical 

 change, but is used more especially when the intention is to stiTdy 

 the nature of the substances decomposed or formed. The expression 

 reaqentjs applied to those substances used for bringing about such 



Analysis and synthesis. These terms refer to two methods of 

 research in chemistry, accomplished by two kinds of reaction, analyti- 

 and synthetical. 



Analysis is that mode of research by which compound substances 

 are broken up into their elements or into simpler forms of combina- 

 tion, and analytical reactions are all chemical processes by which the 

 nature of an element, or of a group of elements, may be recognized. 



