18 Conductivity of Aqueous Solutions. Part II. 



The process employed for making the quartz cup is as follows: A 

 crystal is selected which is perfectly clear and free from imperfections. 

 A slice, in thickness a little greater than the height of the finished cup, is 

 then sawed out at right angles to the main axis. For this operation a thin 

 tinned-iron disk, whose edge is charged with diamond powder, is rotated 

 in the lathe ; and the piece, supported on a sawing table, is pressed lightly 

 against the saw by hand, a wet sponge being held against the edge of the 

 saw with the other hand. This operation of sawing is discussed at some 

 length by Threlfall.* It is both easy and rapid if the saw is in good con- 

 dition. Care must be taken in this and the subsequent operations that the 

 work is not crowded too hard against the abrading surface, as this causes 

 a local rise of temperature which may crack the crystal. A hollow drill, 

 whose internal diameter is but little greater than the external diameter of 

 the finished cup, is then run through the crystal piece at right angles to 

 the sawed surfaces. Such a drill consists merely of a tinned-iron tube pro- 

 vided with a slit running lengthwise, and mounted so that it can be rotated 

 in the lathe. The outer end of the tube is turned off square and is then 

 charged with diamond powder. A small piece of wet sponge is then placed 

 in the tube. Powdered carborundum can be used in place of diamond, 

 and, although it is somewhat slower in starting, it appears to be equally 

 satisfactory afterwards. The core is then taken from the drill and the 

 ends of the cylinder are ground down flat ; for the saw has left them some- 

 what irregular. This operation of grinding is conveniently carried out by 

 means of a carborundum wheel rotated in the lathe, the wheel being kept 

 wet by holding a sponge against it. Before grinding either surface, its 

 bounding edges must be ground off (beveled) ; otherwise the edges will 

 break out irregularly. To cup out the cylinder, a hollow drill, whose 

 external diameter is but little less than the internal diameter of the finished 

 cup, is then run into one end to a depth almost equal to that of the desired 

 cavity. The core which is left from this drill is too strong to be broken 

 out without danger of injuring the outside of the cup; so another, smaller 

 one is next run in to the same depth as the first and concentrically with it. 

 This leaves two fragile pieces, a small rod and a thin tube, which are easily 

 broken out. The cup is next mounted so that it can be rotated in the lathe. 

 This is best accomplished by fastening it with stick shellac to the end of a 

 brass rod held in the lathe chuck. To hold firmly, the crystal must be 

 heated above the melting-point of the shellac; this can be done safely by 

 flashing it with a gas flame. The inside of the cup is then ground to its 

 final diameter and the bottom made flat by using carborundum powder 

 upon the end of a brass rod which is a little less in diameter than the cavity 

 and whose end is squared off, the rod being best held in the hand. The 



*On Laboratory Arts, pp. 187-189. 



