174 ON EMPL ASTRUM PLUMBI. 
at that degree of temperature, it gradually became darker — 
the yellow colour, of the litharge giving way to a reddish 
hue. As the operation proceeded, the mass assumed a pe- 
culiar soapy appearance, the red hue gradually disappeared, 
until after about two hours boiling a plaster was formed, 
which upon being removed from the fire, and allowed to 
stand for a short time, became covered with a bright red 
froth, while the plaster below was of a dull brownish colour, 
and not white as when litharge alone is used. The red 
lead employed in this case failed to perform the desired 
effect, the residue or sediment of metallic lead falling to the 
bottom amounting to about one-fortieth of the weight of the 
litharge used in the process. I would add that the plaster 
formed with this kind of litharge is generally of a lighter 
colour, and more transparent when melted than that made 
with litharge, which produces no such precipitate ; and 
probably this occurs, from the fact of the lead assuming a 
floculent form, and acting in the same way that gelatin does 
in clarifying liquors, — carrying down with it any particles 
of foreign matter that exists in the melted mass. 
5th. Five pounds of English litharge in fine powder was 
mixed with one gallon of olive oil, two pints of water being 
added, as directed by the U. S. Pharmacopoeia. The boil- 
ing was carried on as directed by that authority; the colour 
of the mass became lighter, as the operation was continued. 
Boiling water was added from time to time, and at the ex- 
piration of three and a quarter hours a plaster was formed 
of good consistence — white, with a slightly greenish tint, 
which is attributable to the chlorophylle, or green colouring 
matter in the oil. The change or variety in colour obser- 
vable in experiment fourth did not occur in this experiment. 
Seven pints of water were necessary to the completion of 
the process. 
6th. The same quantity of materials (the American lith- 
arge being substituted) as in experiment fifth, were mixed 
