196 ^ THE AMEKICAN MONTHLY [August, 



" I. See which cement has the greatest angular, vitreous par- 

 ticles. 



" 3. Notice the transparency of the particles. 



" 3. Notice the amount of foreign matter present." 



His observations are : 



I. Buckeye Cemerit. — "As will be noticed, the particles are 

 sharp and clearly defined, and apparently of homogeneous tex- 

 ture ; all the particles, whether large or small, have the same 

 general appearance. By a proper manipulation of the microscope 

 it may be shown that the mass has been completely vitrified. 

 The vmiformity in the appearance of the particles shows that 

 either there are no foreign substances present or that they are not 

 in sufficient quantity to be visible. Up to the present time this 

 cement has given the best results of any that have been tested in 

 the university laboratory." (vSee attached table givino^ compara- 

 tive values from crushing briquettes.) 



II. Gibbs' Portlatid Cement. — " The general appearance is 

 much the same, and at first sight might be said to be identical 

 except for the diflerence in the distribution of the particles, which 

 is, of course, purely accidental ; but if we look closer we will see 

 that the particles are not quite so clear as in the preceding case, 

 and that there are quite a number of minute rounded particles 

 which are entirely difi'erent in appearance from the particles 

 which surround them. These are probably small beads of glass, 

 formed during the burning process, from free silica existing in 

 the clay from which the cement is made. This, as is well known, 

 exerts an injurious effect on the quality of the cement, by intro- 

 ducing an inert foreign substance which interferes with the set- 

 ting of the cement. The slight opacity of the particles of the 

 cement may be due to the admixture of some foreign substance, 

 or to some fault in the burning." 



III. The Sozith Bend Cetnent. — "• This appears to be almost 

 identical with the one last described, except, j^erhaps, that some 

 of the particles are rather coarse. Both the Gi]:)bs' and South 

 Bend cements have given excellent tests, although not equal 

 to the Buckeye." 



IV. '•'• Rosendale cement., which, according to the tests made 

 last year, has the greatest tensile strength of any of the natural 

 cements. As we should expect, it is very different in appearance 

 from any of the aiiificial cements we have just described. The 

 fact that it has not been completely vitrified will account for the 

 opacity and indistinctness of the particles ; still it is quite homo- 

 geneous in appearance and is much more vitreous in structure 

 than any of the other natural cements." 



V. Utica Black Ball^ and VI. J. Clark's ITtica Cctnents. — 

 "These are almost identical in appearance, consisting of opaque 

 masses, surroiuided I)y numerous minute glassy particles. Some 

 of these particles are rounded and are probably similar to those 

 we have already noted. There are others, liowcvcr, with a 



