254 THE POPULAR SCIENCE MONTHLY. 



is probable that the silicates also recombine with some of the 

 water. This is the first step, and produces the so-called setting. 

 It is best passed through while the cement is exposed to the air, 

 and is the reason why cement mixtures must be used as soon as 

 moistened. But, this now complete, a more complex process is set 

 up. The moistened cement brought in contact with the air, or 

 exposed to water, at once begins to absorb carbon dioxide, for all 

 ordinary air contains the gas, and most water holds it in solution. 

 The gas unites with the lime to form a carbonate again, and this 

 goes on until the whole of the lime is turned back to limestone. 

 The same change occurs in the magnesia, but in this the action 

 proceeds more slowly. With a pure lime cement this action is 

 probably nearly complete at the end of a few months ; but, with a 

 cement containing magnesia, it will continue for many years. 

 The strength of the cement increases so long as the change con- 

 tinues. So a Portland cement will develop its full strength in a 

 few months, while our natural cements will not for years, and, 

 so long as it continues, the structure improves. 



Rough testing of cement, so as to enable a workman to get a 

 crude and imperfect idea of its value, is easy. Enough of the 

 pure cement should be taken to make a ball an inch in diameter 

 and mixed with just sufficient water to make it mold readily and 

 be rolled into a ball. Then it should be exposed to the air and 

 left for two hours. At the end of that time it should be set ; then 

 it should be put into water and left. It should grow gradually 

 harder, and should show no signs of cracking or crumbling, 

 even when left for ten days. Any cement that does not endure 

 this test is not of sufficiently good quality to make satisfactory 

 structures ; any cement that stands this properly will be gener- 

 ally satisfactory if properly used. 



In determining how to construct a building, a series of tests is 

 often required that shall show tensile, breaking, twisting, and 

 crushing strength, and also adhesion of the materials used for 

 mortar. No one of these can be dispensed with, since material 

 that will endure one satisfactorily will often fail utterly in an- 

 other, and hence prove worthless for the use desired ; but for gen- 

 eral purposes the test of cement which is the most valuable is 

 that which determines its tensile strength. Comparative tests of 

 this show the value of cements from different sources better than 

 any other one test. 



To make an accurate test of any lot of cement, great care is 

 necessary in selecting and manipulating the samples. The test 

 sample ought not to be taken from a single package, but from 

 several in equal quantities and thoroughly mixed. The sample 

 must also be carefully protected from air and moisture until the 

 test is made. When used, it must be molded with just the right 



