MANIPULATION. 41 



this part place a piece of rubber tubing, and force this rubber-covered 

 part in the manner of a rubber stopper into the larger tube. 



To Cut or Bore Rubber. This is much easier if instrument and rubber 

 are kept wet, either with water or, much better, with solution of potash. 



To Bore Corks. This is done by use of the usual brass cork-borers 

 (a sharpener for these should often be used); round files may be used for 

 some odd forms or sizes. 



To Improve Corks. Corks should always be softened with the cork- 

 presser (for which the rotary forms are particularly good) before using, 

 for both their power of holding in the bottle and of stopping passage of 

 liquids is thereby increased. 



To Make Ordinary Corks Air-tight. To prevent air-passage it is 

 better to use rubber corks, but ordinary corks may be made fairly imper- 

 vious to air by boiling them with paraffine or smearing them with vase- 

 line. 



To Test the Tightness of Joints to Gas-passage, etc. (see Fig. 71. 

 Prepare a glass tube i cm. or less in outside diameter, and about 1 10 cm. 

 in length. Bend 6 or 8 cm. at one end around in a U curve until it is par- 

 allel with the remainder, and to this attach by a stout rubber tube 3 cm. 

 long, tightly wired on, a glass stop-cock ; to the other end fix similarly 

 a glass funnel. Set the whole firmly in a fixed vertical position, funnel 

 upwards. The joint, tube, or other part to be tested is then attached, by 

 a stout rubber tube (wired at both ends), to the stop-cock tube, and mer- 

 cury is poured through the funnel until the long tube is filled ; the mer- 

 cury will then exert about an atmosphere of pressure upon the air in the 

 part being tested, and this pressure may be left on for any desired time. 

 To remove the part, close the stop-cock and place a dish beneath the tube 

 to catch escaping mercury ; the joint may now be loosened and the part 

 removed above the stop-cock. Joints which are often tight to such 

 pressure for a few hours or days, often yield to it afterwards, particularly 

 in the case of waxed -stop-cocks. In comparing readings of the compres- 

 sion at different times, allowance must be made for expansion of the com- 

 pressed air under changes of temperature. 



To Cleanse and Sterilise Germinators, Flower-pots, etc. They may 

 be cleansed by rubbing in water with a snff brush inside and out. For 

 very delicate work they may be boiled first in water containing a weak 

 solution of potash, and next in water containing a little hydrochloric acid, 

 and next in pure water. They may be sterilized by dry steam in a 

 sterilizer of the Arnold type, or, of course, by boiling in water. 



To Clean Mercury. An excellent method * (Fig. 7) is to keep it in a 

 vessel which has a glass stop-cock below from which it may be drawn off 



* Given by Arthur in the Botanical Gazette, 22, 471. He also figures an excel- 

 lent receptacle for holding the mercury. See also Detmer, 42, 43. 



