Were no ammonia present, but only tribasic acetate of lead, as in the case of the earlier methods of precipitating white lead, unless a very slow current of carbonic acid gas were passed through, some of the first formed basic carbonate of lead would be in danger of being over-carbonated and losing its basicity, being converted into mono-carbonate of lead. This danger is much lessened, and consequently a much greater possibility of rapidity of carbonating is conferred, in the case of the process as above described, and by virtue of the ammonia which is present. Nevertheless the carbonic acid gas must not be passed, even into the ammoniacal liquid containing the tribasic acetate of lead, with such rapidity that distinct alkalinity to the usual tests ceases to be maintained, and the process must be terminated whilst the liquid is still alkaline. If a little lead salt on the one hand, and a little ammonia on the other, be left in the mother liquors ultimately obtained on filtering from the white lead, they will be returned and circulated. Fig. 27 shows an elevation (partly in section) of the apparatus employed. _a_ is an iron vessel made of boiler plate, and lined internally with lead. It may be here added that all the apparatus is thus lined, or is constructed of material invulnerable to the action of lead salts, ammonia, or acetate of ammonia. The vessel _a_, which is termed the digestor, should be furnished with, a thermometer, pressure gauge, safety valve, charging hole _b_, and suitably arranged sampling pipe with cock. [Illustration: Fig. 27.--MACIVOR’S WHITE LEAD PLANT.] Agitation of the contents of the vessel is effected by means of the circulating pump _c_, which, drawing off supernatant liquor along with air and ammonia vapour from the upper part of the digestor _a_, by the pipe _d_, forces it through the pipe _e_ to the heater _f_, consisting of a coil suitably heated by steam in a steam vessel _g_, or by similar means, and then is forced through a pipe _h_, leading to the bottom of the digestor _a_, where it terminates in a cone-spreader _i_, provided with a regulating valve _j_. By means of this spreader, the liquid is forced through the heavy and dense mixture containing the oxide of lead, which it vigorously agitates and keeps more or less in circulation as indicated. The charges of litharge and acetate of ammonium will vary according to the strength of the latter. For a 5 per cent. strength of acetate of ammonium solution, however, it will be best to calculate the proportions as follows, viz. about 1200 gallons of 5 per cent. acetate of ammonium liquor for 1 ton of litharge, which should be finely ground.