208 E. M. Walton—-Liquefuction and Cold produced 
Evidences of chemical change.-—When salts capable of meta- 
thesis are mixed, in addition to liquefaction, change of color, 
formation of an insoluble compound, and escape of a gas are 
proofs of chemical reaction. 
An important difference sometimes noticed between mix- 
tures of salts in the solid and the liquid form is the escape, in 
the former case, of some gas, as O,H,O,, CO,, HCl or NH,. 
The gas is dissolved by a liquid solution and eludes observa- 
tion. 
Classification.—Cases of liquefaction may be divided into 
two classes ; the first.including those in which there is mutual 
exchange of base or acid; the second, those in which there is 
no interchange. 
e mixture of lead nitrate with sodium carbonate is an 
example of the first class. There is metathesis, and we obtain 
lead carbonate, sodium nitrate, and ten equivalents of free 
water. 
Pb(NO,), + Na,CO,. 10H, 
— PbCO, + 2NaNO, + 10H,0. 
Hydrated product,—W hen iron nitrate is mixed with calcium 
chloride thirty-six equivalents of water in some form are ob- 
tained. 
= Fe,Cl, + 3Ca(NO,), + 36H,0 
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,(NO,),. 18H,O + 3[CaCl,. 6H,0] 
= Fe,Cl, . 12H,O* + 3[Ca(NO,), . 4H,0] + 12H,0. 
Fe,(NO,),. 18H,O + 3[CaCl, . 6,0] 
. 
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 
* Tt was found that FeCl, . 1211.0, and not Fe,Cl. . 6H,0, was formed. 
