66 Laboratory Arts 



perfect freedom from file scratches, a fine file may be grasped 

 at each end, and pulled backwards and forwards across the 

 material, the file thus working at right angles to its true cutting 

 direction. This is known as " draw filing," and under this treat- 

 ment the surface rapidly becomes smooth and polished, the result 

 being much superior to that produced by use of emery-paper. 

 This latter should only be used wrapped round a file or other 

 similar plane support, in order to keep the surface true. 



Files may be filled with chalk, and then used upon metals 

 in finishing. This method of use is principally of value in 

 dealing with iron or steel. 



A fine surface having been produced, it may be polished 

 with flour emery and lubricating oil spread upon brown paper 

 supported upon a true surface, and finally with rouge and water, 

 or rouge and olive oil. 



This, however, applies only to plane surfaces, curved ones 

 not being amenable to such treatment. Curved surfaces are 

 treated with similar materials while being rotated in a lathe 

 if possible, otherwise they are treated by hand as well as 

 may be. 



Brass, gunmetal, and platinum may have their surfaces 

 "finished" by burnishing, an easy and pleasing process. 

 Having brought the surface " up " as far as possible by files 

 and emery, a smooth steel or agate burnisher is rubbed with 

 considerable force along the surface producing a bright line ; 

 this is repeated until the whole surface becomes bright, the 

 marks of the tool being gradually removed by repeatedly cross- 

 ing the direction of its motion. Platinum crucibles and dishes 

 should be filled with plaster of Paris before this treatment, when 

 an annual burnishing will be found to lengthen their life con- 

 siderably, and to render them less liable to attack from a car- 

 bonizing bunsen flame. 



VI. MISCELLANEOUS NOTES. 



Wood or metal screws that have become too firmly fixed through 

 age to be withdrawn in the usual way, are frequently loosened by 

 holding a red-hot poker to their heads for a minute or so. After 



