TiCHBORNE — On Action of Heat upon Hydrated Salts. 251 
manalytic point. In the second case, the increase in the relative 
volume of water retards the dissociation of water of hydration. In 
the second case it is inversely to the amount of dilution. Thus a 
solution formed of equal weights of crystals of ferric chloride and 
water is not greatly affected by heating to 100° C, while no 
solution of chloride of cobalt weaker than five per cent, is affected 
at the boiling point of water. The thermanalytic point may be 
determined approximately by taking a capillary tube containing 
the salt to be tried, and putting this into a second tube con- 
taining a mixture of chloride of calcium and glycerine, which will 
carry the temperature up to 200° C, or a pressure of about 16 atmo- 
spheres. Heat is now carefully applied, at the same time that the 
temperature is determined by a thermometer, which can be used to 
keep the whole moving. In the following Table a series of observations 
made with chloride of cobalt is placed in juxtaposition with one made 
with ferric chloride, to illustrate what has been just said : — 
Dissociatio7i attended with the for- 
viation of a Basic Precijntate 
Fe^Cl^. 
Dissociation of Water of Hydration CoCl^. 
Per centage of 
Crystals. 
Temperature to 
determine 
Precipitation. 
Per centage of 
Crystals. 
Temperature and Colour. 
60 
10 
5 
Over 100° C. 
No precip. 
94° C. 
82° C. 
50 
25 
10 
( 60° Amethyst. 
< IV Purple. 
( 100° Blue. 
? 85° Amethyst. 
X 124° Purple. 
135° Blue. 
f 180° Amethyst. 
\ 195° Purple. 
i 227° Blue. 
Plate XIX. is a graphic representation of the results contained in 
this Table. Two of the curves represent the dissociation of water and salt 
molecules in hydrated chloride of cobalt ; and two curves, namely, those 
of ferric chloride and the ammonio -ferric alum represent dissociation at- 
tended by separation of basic compounds. The cobaltic curve marked 
^' Amethyst" represents the temperatures at which the dissociation 
begins, which is shown by the pink salt changing to an amethyst 
colour ; the second shows where the salt becomes anhydrous, which is 
indicated by the solution becoming blue. In decompositions of this 
kind dilution retards dissociation : where basic compounds are separated 
it is the reverse — dilution assists decomposition. 
The dissociation, therefore, of the water of hydration of salts when 
in solution is gradual, and progresses with the increment of tempera- 
