OF ARTS AND SCIENCES. 249 



grain of polished crown-glass. Subsequent observations have not 

 entirely confirmed the views expressed at that time. Siill, there does 

 not seem to be much doubt but that certain kinds of glass are capable 

 of receiving perfect lines only in one direction. When the lines are 

 ruled at an angle with the general direction of the grain, the edges at 

 once become serrated if they are very fine ; whereas, if they are coarse, 

 they either become enlarged throughout their whole length, the edges 

 remaining smooth, or else they wholly break up, presenting a very 

 ragged appearance. If the lines are as fine as 25,000 or 30,000 to the 

 inch, this delicately serrated appearance can be detected at once ; 

 whereas, if the lines are coarse, several days may elapse before the 

 tension by which the particles seems to be held together, is broken. 



Two instances occurring in my own experience may serve to 

 illustrate this action. In one, while I was examining a set of lines 

 some days after they were ruled, I was fortunate in seeing two or 

 three lines enlarge throughout their whole length. From being fine 

 lines, they became, almost in an instant, very heavy lines, smooth, 

 black, and of excellent quality every way. The action of breaking 

 up was just slow enough to enable me to follow it. In the other, 

 the lines had been ruled about two weeks ; and for protection they 

 were covered with microscopic glass, closely cemented to the surface. 

 During my examination, the whole surface became completely broken 

 up. Such was the force of the explosion that particles of glass -xoVrr 

 of an inch in length were driven a distance of yJo of an inch. In 

 fact, the debris covered the whole surface of the glass under exam- 

 ination. All the particles presented a curved appearance ; and, with 

 hardly an exception, the curvature was always in the same direction. 

 On both of these plates lines were afterwards ruled in an opposite ■ 

 direction, but without noticeable results. 



It may be said that this phenomenon was due to the peculiar action 

 of the cutting crystal with respect to the surface of the glass ; but in all 

 subsequent experiments, in which similar but less stiiking results were 

 noticed, lines were ruled in both directions at the same time, and 

 under the same conditions. In the case of a particular importation of 

 polished crown-glass from Chance & Sons, the evidence of grain is so 

 marked and of such constant recurrence that all the large plates have 

 been cut into slides 3X1 inches, in the direction indicated by the 

 observations. In general, the direction of the grain can be detected at 

 once by the appearance of lines as fine as 30,000 to the inch, while 

 coarse lines may retain their initial character for several days. 



The present indications are that the grain is only surface deep, and 



