270 REPORT OF NATIONAL MUSEUM, 1899. 



A. H. HEATH. A Manual of Lime and Cement. 



London, 1893, 215 pp. 

 G. R. REDGRAVE. Calcareous Cements: Their Nature and Uses. 



London, 1895, 222 pp. 

 URIAH CUMMINGS. American Cements. 



Boston, 1898, 299, pp. 

 CHARLES D. JAMESON. Portland Cement, its Manufacture and Use. 



New York, 1898, 192 pp. 

 BERNARD L. GREEN. The Portland Cement Industry of the World. 



(Reprinted from Journal of the Association of Engineering Societies. XX, 

 June, 1898). 



PLAYING MARBLES. At Oberstein on the Nahe, Saxony, playing 

 marbles are made in great quantities from limestone. The stone is 

 broken into square blocks, each of such" size as to make a sphere the 

 size of the desired marble. These cubes are then thrown into a mill 

 consisting of a flat, horizontally revolving stone with numerous con- 

 centric grooves or furrows on its surface. A block of oak of the same 

 diameter as the stone and resting on the cubes is then made to revolve 

 over them in a current of water, the cubes being thus reduced to the 

 spherical form. The process requires but about fifteen minutes. 



LITHOGRAPHIC LIMESTONE. For the purpose of lithography there 

 is used a fine-grained homogeneous limestone, breaking with an imper- 

 fect, shell-like or conchoidal fracture, and as a rule of a gray, drab, or 

 yellowish color. A good stone must be sufficiently porous to absorb the 

 greasy compound which holds the ink and soft enough to work readily 

 under the engraver's tool, yet not too soft. It must be uniform in 

 texture throughout and be free from all veins and inequalities of any 

 kind, in order that the various reagents used may act upon all exposed 

 parts alike. It is evident, therefore, that the suitability of this stone 

 for practical purposes depends more upon its physical than chemical 

 qualities. An actual test of the material by a practical lithographer 

 is the only test of real value for stones of this nature. Nevertheless the 

 analyses given below are not without interest as showing the variation 

 in composition even in samples from the same locality. 



