BatLey. | Chemistry of Plasters. 165 
If in the process of heating the temperature was too high, 
some of the carbon dioxide would be driven off, as in the heat- 
ing of limestone to make caustic lime, but the temperature is 
ewefully regulated and not allowed to go above 400° I. It 
might seem that there would be a little overheating, but in the 
presence of the large quantity of water the temperature is kept 
down, and will be low as long as water is driven off. It should 
be remembered that about five per cent. of water still remains, 
and this would be driven off at a higher temperature or if the 
heating were continued beyond the critical period. So, till this 
last portion of water is expelled, the carbon dioxide will not be 
attacked. That this is practically true is shown by the series 
of analyses. The amount of carbon dioxide in the first sample 
was 3.02 per cent. and in the last 3.55 per cent. 
Another complete set of analyses of the material, taken Au- 
gust 27, 1897, as it went through the mill, was made. The crude 
material at this plant was also gypsite. In this case the 
amount of carbon dioxide was not determined, but the results 
are from the calculated quantities. The samples selected were 
as follows: 
No. 1, crude material. 
No. 2, sample taken after two hours of boiling. 
No. 3, the finished material. 
No. 4, a sample that has been set. 
No. 5, the screenings or tailings. 
No. 6, a sample that has been treated with a ‘‘ retarder.’’ 
The composition of each sample is as follows: 
COMPOSITION OF PLASTER AT DIFFERENT STAGES OF 
MANUFACTURE. 
Silica and insoluble residue ...| 9.78 | 10.84 | 12.02 | 10.84 | 25.83 IAL 3 
Tron and aluminum oxides....| 0.30 0.50 0.44 0.42 iL GY 0.67 
(CAWICHUTIN OHACO> basis dso soos eade BOG | 3B.G9) | B.A! | SOB) B9)Gx B45) OY 
Magnesium oxide............. 0.33 0.52 0.44 0.39 0.95 0.44 
Sulphuric anhydride.......... Bool) | 40.6% | 4021 | B20) | By) 40.94 
Carbon dioxide (calculated)...| 3.06 4.70 6.82 HO | IL Gr 5.85 
NIV CHEST OY ar clas ean DE Sten ean UY BU 9.02 4.98 | 16.35 4.18 4.78 
