SIVAN (GAOL IE. 141 
usually in very small amount, which had no opportunity of being acted 
upon chemically by the basic cement mixture. ‘The coarse quartz grains 
can, however, be easily distinguished by their gritty feeling, or definitely 
proven by means of a magnifying glass or microscope.’ For good Port- 
land cement this insoluble residue, free from carbon, should be prac- 
tically nothing ; any appreciable amount should arouse our suspicion as to 
the quality of the cement. But this is not invariably true. There may 
be mixtures which, when burnt, contain some insoluble quartz, yet in 
these it should be carefully determined and allowed for in the calculation 
of the batch. Still, such cases are uncommon, and the above caution 
-must be allowed to stand. In all cases finer grinding will cause the 
insoluble residue to disappear. 
Supplementary Physical Methods.—Of physical methods of exam- 
ination which might be employed we will mention only two, the deter- 
mination of the heat of hydration of calcium oxide and microscopic 
examination. 
It has been suggested to calculate the amount of calcium oxide or 
carbonate present in a limestone by heating it over the blast lamp so as to 
expel all of the carbon dioxide and then determine the heat of hydra- 
tion in a calorimeter. Knowing that one gram of pure calcium oxide 
on hydration evolves 269.6 calories, one might suggest that by the use 
of a suitable factor, obtained by a number of gravimetric analyses for a 
given limestone, the amount of calcium might be calculated. In a great 
number of experiments made with limestone, using the calorimeter and 
calcareous mixtures for other purposes of investigation, the Ohio Survey 
has shown that so many factors affect the results obtained that it would 
not be advisable to employ methods based on this heat reaction, espe- 
cially as the time saved is not worth the trouble. 
Conditions are different, however, with the examination of burnt 
cement, where it is desirable to judge the material with reference to the 
uncombined lime present, so as to be able to determine the time of curing 
and other important facts. Here the calorimeter becomes a valuable 
instrument whose intelligent use will bring to light important data. The 
application of the calorimeter for this purpose will be more fully discussed 
under the head of the testing of Portland cement. 
Microscopic examination, although hardly ever made absolutely 
necessary, frequently becomes a helpful aid in detecting the presence of 
foreign substances in a cement, like carbon, underburnt cement or slag, 
which can be distinguished by the difference in color and general appear- 
ance. In the mechanical analysis it enables one to make measurements of 
particles from sediments. In working with a cement which has a ten- 
dency to dust, or show abnormal behavior, the microscope may be able 
to show the cause of the trouble by proving the presence of non- 
hydraulic crystal'ine bodies. 
