ATTRACTION. 155 



The acetate of lead is decomposed by sulphuric acid ; the nitrate 

 of silver, by copper ; nitrate of copper, by iron ; nitrate of mercury, 

 by copper ; muriate of soda, by sulphuric acid ; and so in instances 

 innumerable. In all these cases, except the first, there is a new 

 salt formed by the addition of the decomposing body, the acid or 

 base of the preceding salt being liberated. 



(CC.) In such cases, therefore, a compound of two principles is de- 

 composed by a third, which unites with one, and excludes the other, 

 which may be thus illustrated; A+B=C. D unites with A, and 

 forms the compound A+D, or with B, and forms the compound 

 D-j-B, B in the first case, and A in the second, being excluded. 

 If any solid appears, it is called the precipitate, and the decomposing 

 body, the precipitant ; the fluid is called the solution. 



(dd.) In some cases, a weaker affinity is compensated by an in- 

 creased quantity of the feebler ingredient. Muriate of soda 2, 

 oxide of lead 1 , there is no effect in twenty four hours ; but with 

 muriate of soda 1, and oxide of lead 3 or 4, decomposition follows in 

 twenty four hours, and muriate of lead is formed, and soda, or its sub- 

 carbonate, evolved ; this fact is the foundation of the manufacture of 

 soda from common salt. The solution of sulphate of copper is blue, 

 but if the muriatic acid is added largely, the color changes to green, 

 indicating a decomposition, and the production of a muriate of cop- 

 per. 



(EE.) DOUBLE ELECTIVE AFFINITY, is WHERE TWO COMPOUNDS, 



EACH CONSISTING OF TWO INGREDIENTS, ARE DECOMPOSED, FORM- 

 ING TWO NEW COMPOUNDS. A, composed of B+C, is mixed with 

 D, composed of E+F; the result may be, B+E, or B-fF, or 

 C-f E, or C + F. Important decompositions, otherwise unattaina- 

 ble, are often effected in this manner. 



(FF.) DECOMPOSITIONS STILL MORE COMPLEX, INVOLVING THE 



ACTION OF SEVERAL AGENTS, EACH CONSITING OF TWO OR MORE 

 PRINCIPLES, MAY PRODUCE SEVERAL DECOMPOSITIONS, AND SEVE- 

 RAL NEW COMPOUNDS. Many of the processes in the animal and 

 vegetable economy, are of this description, and some among minerals. 



(GG.) Extraneous circumstances and forces influence chemical ac- 

 tion, among which the chief are quantity, cohesion, insolubility, grav- 

 ity, elasticity, efflorescence, temperature, mechanical pressure, and 

 electricity. 



1 . Quantity of matter exerts an important influence on chemical 

 decompositions. This is a well known practical fact. In dissolv- 

 ing a salt in water, the first portions added, are more readily dissolv- 

 ed than subsequent ones, and the energy of attraction* diminishes as 

 we approach the point of saturation. 



* Or is it mechanical obstruction that retards the solution ; the degree of affinity 

 remaining the same ? (Communicated.} 



