produced by the mutual Reaction of Solid Substances. 293 



Having mixed equivalent weights, the product was dried on 

 the smooth surface of a plate of plaster of Paris, which absorbed 

 the moisture ; and an analysis showed the two new salts ob- 

 tained to be hydrated. 



Therefore 



Fe 2 (N0 3 ) 6 . 18H 2 + 3[CaCl 2 . 6H 2 0] 

 = Fe 2 Cl 6 . 12H 2 0* + 3[Ca (N0 3 ) 2 . 4H 2 0] + 12H 2 0. 



This experiment was repeated with a mixture of Fe^NO^ . 

 18 H 2 with NaCl, also of Ca(N0 3 ) 2 . 4H 2 with MgS0 4 . 

 7 H 2 ; and the first product was found to contain iron chlo- 

 ride, the second nitrate of magnesium, both in the hydrated 

 form. 



At the more or less low temperature due to liquefaction, 

 there is naturally a tendency for salts to crystallize out from 

 the saturated solution. 



The crystalline character is sometimes perceptible to the 

 senses; for the product often contains grains much coarser 

 than did the finely-powdered salts first taken. 



Effect of Temperature. — Experiments show that sometimes 

 liquefaction takes place readily at a temperature somewhat 

 elevated, but not at all at a low temperature. A mortar and 

 pestle which had been warmed by hot water were occasionally 

 used, care being taken that the heat should not be great enough 

 to cause either of the original salts to melt in their water of 

 crystallization. 



When two salts capable of metathesis are mixed, chemical 

 action apparently begins immediately at every point of con- 

 tact. But there is a limit to the fineness of division which 

 may be effected by mechanical means; and the substance 

 consists of minute grains coated on the outside with the new 

 product, while remaining unchanged at the interior. 



When liquefaction ensues, the interchange is continued, 

 either because by removing the particles of the product new 

 surfaces are presented, or because the liquid, penetrating the 

 granules, separates them into their molecules. 



If the salts taken furnish little or no water in excess of that 

 required to combine with the new ones formed, the process of 

 interchange apparently soon ceases, unless sufficient heat is 

 supplied to prevent the constituents of the product from assu- 

 ming the solid form. 



Difficultly Soluble Salts. — When salts difficultly soluble are 

 used, moistening follows but slowly, if at all. The molecules 



* It was found that Fe a Cl 9 . 12 H 2 O, and not Fe a 01 e .6H a O, waa 

 formed. 



