s of either
article, which contain that impurity, rejected.

Lime, bog ore, and various metallic salts, such as ferric chloride,
barium chloride, and copper sulphate have been suggested, and in some
instances are used instead of aluminium sulphate, but the latter is
generally employed.

_Evaporation to Crude Glycerine._--The clear treated lyes, being now
free from fatty, resinous, and albuminous matter, and consisting
practically of an aqueous solution of common salt (sodium chloride) and
glycerine, is converted into crude glycerine by concentration, which
eliminates the water and causes most of the salt to be deposited.

This concentration was originally performed in open pans heated by fire
or waste combustible gases. In the bottom of each pan was placed a dish
in which the salt deposited, and this dish was lifted out periodically
by the aid of an overhead crane and the contents emptied and washed.
Concentration was continued until the temperature of the liquor was 300 deg.
F. (149 deg. C.), when it was allowed to rest before storing.

This liquor on analysis gave 80 per cent. glycerol and from 9 to 10-1/2
per cent. salts (ash); hence the present standard for crude glycerine.

Concentration in open pans has now been superseded by evaporation _in
vacuo_. The subject of the gradual development of the modern efficient
evaporating plant from the vacuum pan, originated and successfully
applied by Howard in 1813 in the sugar industry, is too lengthy to
detail here, suffice it to say that the multiple effects now in vogue
possess distinct advantages--the greatest of these being increased
efficiency combined with economy.

The present type of evaporator consists of one or more vessels, each
fitted with a steam chamber through which are fixed vertical hollow
tubes. The steam chamber of the first vessel is heated with direct
steam, or with exhaust steam (supplied from the exhaust steam receiver
into which passes the waste steam of the factory); the treated lyes
circulating through the heated tubes is made to boil at a lower
temperature, with the reduced pressure, than is possible by heating in
open pans.

The vapour given off by the boiling liquor is conveyed through large
pipes into the steam chamber of the second vessel, where its latent heat
is utilised in producing evaporation, the pressure being further
reduced, as this second vessel is under a greater vacuum than No. 1.
Thus we get a "double effect," as the plant