38 ANNUAL REPORT 
soft, marl-like dolomitic materials. But the relative proportions between 
the magnesium and calcium carbonate vary greatly, so that we start from 
practically all calcium carbonate to the dolomitic proportions 54.35 calcium 
to 45.65 per cent. magnesium carbonate. The specific gravity of dolomite 
is from 2.8 to 2.95; the hardness of the compact rock is greater than that 
of limestone and it is distinguished from the latter in that it is much less 
readily soluble in cold hydrochloric acid. Some of the hard dolomites show 
a distinct fissured structure. Sometimes dolomite is found in beds formed 
by direct deposition as the precipitate from saturated water, or it may be 
found replacing original deposits of limestone. As a cement material it 
is not promising, since it gives rise to two silicates (of lime and magnesia) 
which have different rates of hydration and which hence interfere with 
each other in the hardening process, unless the burning took place at a low 
temperature not over 1000° C. In nature we find dolomitic hydraulic 
materials in the form of the natural cement rocks, like the Rosendale, 
Louisville, Utica and Milwaukee. But all of these must be burnt at a 
low temperature, at which not all of the calcium carbonate has been | 
decomposed. 
MAGNESIA. 
This substance is chiefly derived from magnesite, magnesium carbon- 
ate, which is closely analogous to calcium carbonate in all its properties. 
It crystallizes as rhombohedral crystals, isomorphous with calcite. Its 
decomposition, however, takes place at a considerably lower temperature 
than that of calcite, carbon dioxide being evolved at a lively rate at about 
500° C. Magnesium exhibits a great tendency to form basic and hydrated 
carbonates, whose compositions vary with the conditions of precipitation. 
On heating magnesium solutions to higher temperatures, subjecting them 
to higher pressures or evaporating them to dryness, the normal carbonate 
is formed. 
The mineral hydromagnesite is a basic carbonate of the composition 
3MeCO,Me(OH),.3H.O. Artificially prepared magnesium carbonates are 
usually bulky, very light precipitates. from boiling solutions a denser 
basic carbonate is obtained than from cold ones. In nature we find mag- 
nesium carbonate as a dense, usually white, mineral. On burning this 
first loses its carbon dioxide and begins to shrink in volume quite decidedly, 
continuing to do so up to very high temperatures. 
Magnesium oxide slakes on contact with water and shows some in- 
crease in volume, but this increase is much less than that of the calcium 
oxide converted into calcium hydroxide. 
The hydration of magnesium oxide is itself associated with hydraulic 
activity; that is, magnesium oxide prepared at a low temperature when 
made up with water will harden and form a tolerably strong cement, as 
was found to be the case by Deville and others. Magnesium silicates and 
aluminates show decided hydraulic properties. Thus Fuchs was able to 
