306 aes ANNUAL REPORT - %, 
cements like those containing excess of lime show a high heat of hydration, 
while the standard Portland cements, as well as cements with a high lime 
content, but a silicious clay base, have a low heat of hydration. As has 
been mentioned repeatedly, the number of calories given off on hydra- 
tion by a cement are not intrinsically a sufficient criterion for judging 
the quality of a cement, but it is a fact that the standard American 
cements do not show a hydration value of over 8 calories per gram in 
excess of water. It has also been found that of cements made by the 
writer those which showed a high heat of hydration were apt to fail in 
the boiling test, though not always. The greatest help of this test is in 
estimating whether or not a cement has been stored sufficiently long. 
Many Portland cements would stand the hot test satisfactorily if they 
were allowed to cure longer, and the calorimeter test would thus tend to 
protect such fresh but otherwise good cements by indicating clearly that 
further storing is necessary. 
The writer added to a sound Portland cement lime ignited to a high 
temperature in a crucible furnace in percentages varying from I to 6 per 
cent. Boiling tests showed no sign of the breaking down of the cements 
up to 4 per cent. On making calorimeter tests the heats of hydration deter- 
mined practically form a straight line, as shown by the accompanying 
curve (figure 75). The abscissa indicates per cent. of caustic lime added, 
which was commercial lime, and the ordinate the calories per gram of 
cement mixture. 
SPECIFIC GRAVITY. 
The specific gravity of ignited Portland cement varies from 3.10 to 
3.25. Storing invariably reduces the specific gravity. It has been fre- 
quently claimed that this constant furnishes a criterion in regard to 
Whether the cement has been burnt properly or was underburnt. But 
extensive experiments have shown that this is not the case, since cement 
burnt just below vitrification has practically the same specific gravity 
as the properly burnt cement. It is of use, however, in estimating 
whether or not a given cement has been cured sufficiently and when this 
has been determined for a certain cement it affords the chemist a valuable 
aid in regulating the time of storing. 
Overburnt cement shows a lower specific gravity than realesabansri 
cement owing to the vesicular structure developed. It becomes evident, 
therefore, that the specific gravity does not deserve the general application 
it now receives except when it is applied to the detection of adulteration. 
Specific Gravity Apparatus.—There are many instruments for the de- 
termination of specific gravity. The accompanying figure represents the 
apparatus designed by F. M. Meyer. The parts a and b are connected by 
means of a loose rubber tube. Through /, about 110 cm, of alcohol (redis- 
tilled over caustic lime) is poured in, lowering the bottle 7. The marks a 
