The charge of monoxide of lead and acetate of ammonia in the vat should be kept constantly stirred by the agitator until it becomes whitish in colour, when it will be found that the monoxide of lead has become converted into hydrate and acetate. The workman will know that this change is complete when the reddish or yellowish appearance of the monoxide of lead disappears, and the mass in the vat becomes whitish in colour. The hollow axis of the agitator is connected with a pipe, by which carbonic acid gas is supplied to it under pressure sufficient to cause the gas to pass through the contents of the vat. This gas escapes at the lower ends of the pipes _g_, and ascends through the liquid in which the hydrate and acetate of lead are held suspended. A free flow of this gas should be maintained, and some excess allowed to pass away by a pipe on the cover into the ammonia catch box _r_. The pipe dips down into the acetic acid which this box contains, and any ammonia passing off with the carbonic acid gas is caught by the acid, and forms acetate of ammonia. When the hydrate and acetate of lead are completely converted into basic carbonate (and this the workman will know by its changed appearance), the motion of the agitator is caused to cease, and the white lead is allowed to settle. There is a gauge glass on the side of the vat _a_, and in this glass the changes of appearance can be recognised by which the workman regulates the process. When the white lead is deposited, the liquor is drawn off into the receiver _i_ by opening the cock _h_. The plug _p_ is then displaced, and the white lead is allowed to descend into the washing cistern _m_. Below the cock _h_ is another cock or cocks, in order that any further quantity of liquor may be drawn off into the cistern _i_ if required. The taper plug _p_ is then raised, by being pushed up from underneath the vat _a_, through pipe _l_. The white lead requires to be well washed with clean cold water. When a fresh charge of monoxide of lead has been placed in the vat _a_, the liquor is pumped up on to it from the receiver _i_, and then a further quantity of solution is drawn from the cistern _b_, and also from the ammonia catch box _r_, until the proper quantity has been supplied, which is determined by the gauge.