236 THE AMERICAN MONTHLY [July 



more or less opal or uncry stall ized silica which contains 

 water, so that analysis shows that chalcedony contains 

 from 0.3 to 2.5 per cent or more of water. 



In this section, when examined in polarized light, the 

 needle-like radiating- crystals of the quartz are seen to form 

 circular areas, in which the successive layers of g^rowth 

 can be well seen. These areas represent in section the 

 rounded elevations of the free surface. Each circular area 

 is composed of a very g-reat number of crystals, which 

 g-ive it the radial character. These areas interfere with 

 each other more or less, but an examination of their 

 concentric banding- as seen with the polariscope ena- 

 bles one to trace out the diiferent stages of the forma- 

 tion of the mineral. At the edges representing original 

 free surfaces soaie well-formed crystals of quartz have 

 developed and encrust the edge of the section. The same 

 region shows the enclosure of foreign matter, giving a 

 banded appearance in ordinary light. This is essentially 

 the structure of agate, which is simply banded (and gener- 

 ally colored) chalcedony. Carnelian or sard is likewise a 

 variety of chalcedony of a red or yellow color (not banded). 



Amos P. Bkown. 



NEW PUBLICATIONS. 



Biolog-ical lectures delivered at the Marine Laboratory 

 at Wood's Holl. Ginn & Co., pp. 188; constitutes a very 

 interesting volume. 



Microscopic Internal Flaws inducing Fracture in Steel. 

 By Thomas Andrews, F. R. S., F. C. S., M. Inst. C. E., 

 etc. 8vo, pp. 52. (London : E. and F. N. Spon. 1896.) 



A paper of considerable import. mce to Civil Engineers, 

 reprinted from P^ngineering-, on Microscopic Internal 

 Flaws in Steel, Railway Locomotive and Straight Axles, 

 Tyres, Rails, Steamship Propeller Shafts, iind Propeller 

 Crane Shafts, and other Shafts, Bridge Girder Plates, 

 Ship Plates, and other Engineering Constructions of Steel. 

 There are 30 micro, figures showing internal defects. 



