MANIPULATION 247 



Calibrating. See Graduating, below. 



Cutting. If under i cm. diameter, file a nick on the desired line of cut; 

 then, grasping the tube firmly with fingers each side of the nick, which is to 

 be turned away from the worker, pull the ends of the tube apart, at the same 

 time snapping it sharply outward in the middle, when it will break across. 

 If somewhat over i cm. diameter, a deeper nick with greater exertion will 

 still permit it to be broken in this way. But if much larger, or if a vial or 

 bottle is to be cut across, another method must be used, such as the follow- 

 ing: File a groove along the entire line of the desired cut, making it deeper 

 in one place; then, pulling the two ends gently apart, bring the nick into 

 the tip of a very fine flame, when the glass will crack along the line of the 

 groove. Another method, especially efficient with large bottles, is this: 

 File a nick upon the desired line of cut, making it taper up at both ends; 

 wrap around the glass two long strips of wet filter-paper parallel with the 

 desired line of separation, but i mm. from it in thin glass and more in thicker; 

 then bring the nick into the tip of a fine flame, when it will crack cleanly along 

 the desired line. Also, a very hot iron point led along the line to be broken 

 is said to be effective. Also, for large tubing a very efficient special cutter 

 may be purchased. If a very short end, too short to be grasped firmly in 

 the hand, is to be cut from a tube, file a nick in the usual way, then rest the 

 tube with the nick (which is to be upwards) over the edge of a file. A smart 

 blow on the small end with a light metal object, such as a key, will then 

 break the end cleanly off. 



Graduating. If only regular interval marks are wanted, these may be 

 well applied by use of the wheel-marker for stems, earlier described (page 

 197). If measured intervals are needed, they may be applied with water- 

 proof India ink by use of a ruler and thread-marker made from a thread 

 stretched by a small wire bow. Or, and for most purposes best of all, a 

 pasteboard or wooden millimeter scale is wired to the tube. Or tubes 

 of injured thermometers (and, better yet, the scales of milk-glass scale ther- 

 mometers) wired to the tubes are excellent. To make the graduation per- 

 manent, mark the lines with a writing diamond, or with a fine file, and rub 

 into the marks some white lead, or some plaster of Paris containing a soluble 

 color, such as carmine, or even waterproof India ink. 



It is not possible to manufacture glass tubing of uniform bore, and hence 

 for very exact work, especially if short lengths of tubing are involved (as 

 when an air column is compressed under several atmospheres), it is neces- 

 sary to calibrate it, that is, to determine and mark the error due to the 

 inequalities. It is accomplished by introducing a short thread of mercury 

 into the tube, pushing it along from point to point, measuring its length at 

 each stage, and marking this upon the tube. If the stage with sliding micro- 

 scope employed for this purpose in physics is not available, a fair substitute 

 is found in the use of an ocular micrometer, the gauge being moved over the 

 stage, while the thread of mercury may be forced along by use of a pipette 

 bulb on one end of the gauge, or, better, by an adaptation of the screw 

 arrangement found in a Reichert thermo-regulator. 



Sealing. If held, sloping downwards, in the hottest part (the tip of the 



