ON LEAD PLASTER. 
273 
the same as when twice or thrice the amount of water is 
used, except that the puffing up is less considerable. 
In every case, by experience it will readily be learned 
when it would be prudent to add a little water to complete 
the preparation. The water is not totally evaporated until 
the plaster sinks down, and slightly fuliginous vapors arise. 
Before speaking of the advantages which this method 
presents, it will not be useless to add that there is one cir- 
cumstance on which success essentially depends. The olive 
oil should be pure : adulterated, as it frequently is in com- 
merce, but an imperfect product will result, and which can- 
not be used, as some of my friends have found to their cost. 
I may likewise add that the quantity of water indicated 
would probably suffice for a larger amount of plaster; but 
in preparing less there would be a risk of burning if the 
water were diminished in proportion to the other ingredi- 
ents. It is evident that for 5, 10, or 15 kilogrammes of 
matter, the evaporation of the water would present but 
slight differences, as the time of the operation would remain 
nearly the same. 
It is recommended to pour upon the plaster, when made 
and hot, some cold water, for the purpose of cooling, 
and to facilitate the kneading : this water dissolves the gly- 
cerine, or sweet principle, produced by the reaction between 
the oil and oxide of lead. Formerly this principle was not 
separated, as not injurious; on the contrary, I think that it 
contributes to give suppleness, the adhesive state of which 
Lemery speaks. This washing would then be injurious. 
If then no water be added, if little or none remain when 
the process is complete, kneading becomes useless ; and in 
all cases is not the kneading improper? Does it serve any 
other purpose than to intermix minutely throughout the 
plaster, air and water ? The presence of these two agents 
would tend to determine part of the rancidity, as is seen in 
fatty bodies> of which the surface remains exposed to the air. 
