ON THE DECOMPOSING POWER OP HOT STEAM. 187 
When potash ahim is calcined, the aUiminous 'sulphate 
parts with its acid, whilst the alkaline salt remains admixed 
with the alumina. Berthier has stated that the continued 
action of heat on this mixture, results in the displacement 
of the sulphuric acid by the albumen, which remains com- 
bined with the alkali. Tighhuan, in carefully repeating 
this experiment, avoiding the contact of aqueous vapour, 
observed no decomposition even at a white heat ; but when 
water was present, even in small quantity, the decomposi* 
tion was rapid ; hence he infers the accidental action of 
watery vapour in Berthier's experiment. 
From the action of aqueous vapour on anhydrous alum, 
Tighlman was lead to infer a similar action would take 
place when its mineral representative, felspar, which is a 
double silicate of alumina and potash, was placed in the 
: same circumstances. On passing steam through a quantity 
of that mineral in small fragments, highly heated, no appa- 
rent effect had taken place, but partial fusion, and a certain 
degree of vesicularity in the portions most exposed. When, 
hoxvever, these were pulverized and boiled in water, the 
concentrated solution was strongly alkaline from the alu- 
minate of potash it held in solution. 
Tighlman infers from the experiments he has heretofore 
made, the lollowing general rule, viz. : Whenever a salt 
from its own elements alone, or by the addition of those of 
water, can produce a volatile acid and a fixed base, the 
evolution of this acid, and the liberation of this base will be 
determined by passing a current of aqueous vapour over the 
salt raised to a high temperature. When either the acid 
or base to be liberated, forms a combination with water which 
can resist decomposition by the heat employed, the ten- 
dency to form such hydrates adds much to the decomposing 
power of the aqueous vapour. Although potash and soda 
are not by themselves fixed bases at high temperatures, yet 
by the use of the substances before mentioned, they can 
