156 University Geological ‘Survey of Kansas. 
some experiments in this line, that there is any direct union 
between the silica and the calcium and magnesium. The ce- 
ment plaster stands, in some respects, between ordinary mortar 
and hydraulic cement, and a much more extended study is nec- 
essary to determine all its properties. | 
In the above analyses the chemist will readily see that the 
sulphuric acid is combined with the calcium, and the remainder 
of the calcium with carbon dioxide, and all the magnesium with 
carbon dioxide. It will be evident that, with this distribution, 
in many cases there is a slight excess in the sum. This would 
usually be the case in the analysis of such material, if the alka- 
lies and other substances, which are in small. quantities, were 
determined. This seems to point to the fact that some of 
the calcium, and probably magnesium, is combined with the 
silica, and is decomposed in the process of getting the mineral 
into solution. This insoluble residue also represents both pure 
silica and some silicates that have escaped decomposition. Other 
experiments confirm this conclusion. These beds seem to con- 
sist of quite pure lime salts, with only a small admixture of 
either magnesium or iron and alumina. | 
The theoretical amount of water in the crude material, if it 
consists of absolutely pure gypsum, is 20.90 per cent.; this 
would agree with the formula CaSO,,2H,O. As the commer- 
cial samples are always impure, the per cent. of water is lower, 
but a comparison shows that it is usually a little higher than 
the theoretical amount for a sample of that degree of purity. 
This would be accounted for by the fact that the air-dried sam- 
ple is used for analysis, and that it retains some of the hygro- 
scopic water. Thus, a sample of crude gypsite that contained 
60.27 per cent. of calcium sulphate should contain 16.41 per 
cent. of water, but the analysis shows that it really contains 
17.05 per cent., so the extra 0.64 per cent. is due to hygroscopic 
moisture. Had the sample been carefully dried for a sufficient 
length of time most of this would have been driven off. It is 
necessary, however, to use great care in the drying of the crude 
material, as water is driven off in small quantities at as low a 
temperature as 80° C., and, in fact, plaster of Paris may be 
