32 REPORT—1843. 
the greatest proportion of oxide of erbium, mixed with oxide of terbium and yttria. 
If a solution of ordinary yttria in nitric acid be treated in the same manner, and the 
several precipitates be heated separately, the first precipitate will give a dark yellow 
oxide, the colour of each succeeding one will be paler and paler, till at last a white 
oxide will be obtained, consisting chiefly of yttria, with a trifling quantity of oxide of 
terbium. In making these experiments it is of importance that the yttria be free 
from iron, uranium, &c., a matter of considerable difficulty. It is therefore better 
to commence precipitating with a weak solution of hydrosulphuret of ammonia, and 
when the precipitate has no longer a shade of bluish green, then to apply the caustic 
ammonia as described. A better method in general is to add a portion of free acid 
to a solution of yttria, and then to drop in a solution of binoxalate of potash, con- 
tinually stirring till the precipitate no longer redissolves. In a couple of hours a 
precipitate will form, which is to be separated, and the remaining solution treated 
as above described, and that as long as any precipitate is formed. If the remaining 
fluid be then neutralized with an alkali, a small quantity of nearly pure oxalate of 
ttria is obtained. Of the precipitates the first obtained are most crystalline, and 
fall quickly, the last more pulverulent, sinking slowly. The former contain most 
oxide of erbium, mixed with oxide of terbium and yttria ; the next contain less oxide 
of erbium, more of terbium and yttria ; while the latter contain more and more yttria, 
mixed with oxide of terbium. The first precipitates are always reddish, and the last 
colourless. Ifa mixture of the oxalates of these bases be treated with a very diluted 
acid, we obtain first a salt containing mostly yttria, then one richer in oxide of 
terbium, and the remainder contains principally oxide of erbium. I have even once 
succeeded in obtaining a double salt of sulphate of potash and sulphate of oxide of 
erbium (which is with difficulty dissolved in a saturated solution of sulphate of potash), 
by treating a somewhat concentrated solution of the nitrates of oxide of terbium and 
erbium with an excess of sulphate of potash. 
That much time and labour have been employed in arriving even at the results 
which I have hitherto obtained, will be evident from the little I have been enabled 
to make known, particularly when it is considered that one or two grains of yttria 
have often been divided into nearly a hundred precipitates, which have been indi- 
vidually examined ; but I live in hopes that the knowledge already obtained will 
soon enable me to publish a more complete account of my investigations. 

On the Heat of Combination. By Dr. ANDREws. 
The object of this communication was to announce the following general principle, 
as a consequence of previous researches of the authur on the same subject, and to 
give a general account of some recent experiments which appear to him to establish 
its accuracy. The law may be thus stated: “ When one base displaces another from 
any of its neutral combinations, the heat evolved or abstracted is always the same 
when the base is the same; or, in other words, the change of temperature which oc- 
curs during the substitution of one base for another in any neutral compound, de- 
pends wholly on the bases, and is in no respect influenced by the acid element of the 
combination.” To test the accuracy of this principle by direct experiment, equivalent 
solutions of various neutral salts were decomposed by the addition of a dilute solu- 
tion of the hydrate of potash. When the strength of the solutions and their tem- 
peratures were properly adjusted, the same variation of temperature always occurred 
during the decomposition of salts of the same base. If the base (in the state of a 
hydrate) developed, when alone, less heat than the hydrate of potash in combining 
with the acids, an elevation of temperature occurred during the decomposition of its 
salts by the latter ; if the reverse were the case, the decomposition of the salts was 
attended by a diminution of temperature. Thus the decomposition of equivalent so- 
lutions of the salts of the oxide of copper was attended by the evolution of the same 
amount of heat, as was also the decomposition of the salts of the oxide of zinc ; but. 
the heat extracted by the former was about twice as great as that extracted by the 
latter, because the oxide of copper produces less heat in combining with the acids 
than the oxide of zinc. The salts of lime furnish an example of an absorption of 
heat when their solutions are decomposed by potash,—a circumstance easily explained - 

