PREPARATION OF THE HYPOSULPHITE OF SODA. 
43 
carbonate which is added to the bisulphite to transform it 
into the neutral sulphite. 
The best process for procuring the neutral sulphite con- 
sists in dividing a solution of alkaline carbonate into two 
parts, saturating one with sulphurous acid gas, and after 
wards neutralizing it with the second portion of the carbo- 
nate. When a solution of carbonate of soda is saturated 
with sulphurous acid gas, the liquid not only contains ali 
the gas forming the bisulphite, but also that which the 
water of the liquid retains in solution. Hence when the 
first portion is neutralized by the second, we do not obtain 
a neutral solution, but a mixture containing excess of the 
bisulphite. It is therefore best to introduce the neutralized 
liquid into a large flask, to boil it alone at first, and not to 
add the sulphur which is to convert it into hyposulphite 
until the excess of sulphurous acid has escaped. In follow- 
ing this plan, almost the whole of the soda is converted into 
the hyposulphite. If requisite, the salt may be purified by 
recrystallization. 
M. Plessy adopts a method of purifying the hyposulphite 
which is also used with great advantage in the preparation 
of pure carbonate of soda from the impure commercial 
crystallized carbonate. The salt is melted in its water of 
crystallization, and the heat is continued so as to evaporate 
a portion of the water. When the solution cools, the 
hyposulphite alone crystallizes, the mother-liquor retaining 
the impurities. If the hyposulphite does not contain any 
sulphate, it will not, when dissolved in a large quantity of 
water, precipitate salts of baryta. 
I may mention an easy process for determining the iden- 
tity of the salt. It is based upon the alteration which it 
undergoes when heated, and consists in ascertaining the 
weight of the residue obtained by calcination, and examin- 
ing if it contains sulphur. From 1 5 to 30 grs. are introduced 
into a tube closed at one end. The tube is then carefully 
heated to drive off the water of crystallization ; and when 
