227 

 ANSWERS TO QUERIES, 



INSERTED AT P^GE 176 IN THE LAST NUMBER. 



"'^ II. — What causes the change of colour in polished steel, while 

 undergoing the process of tempering ? '' 



The change of colour produced on the polished surface of 

 steel or iron, by the application of heat, is occasioned by a par- 

 tial oxidation of an extremely thin film of the metal, in conse- 

 cjuence of its combination with the oxygen of the atmosphere, for 

 which it has a great affinity ; which is moreover increased by the 

 application of heat. The first change observable on the bright 

 surface, is that of a pale straw-colour, which gradually becomes 

 darker, with a shade of brown ; this, by a further increase of 

 temperature, becomes gradually darker, until it assumes the 

 colour of a deep blue or purple. If the heat be continued, this 

 blue colour loses its brilliancy, and at length gives place to a dull 

 lead-colour ; about which time the metal begins to get red hot ; 

 after doing which, the film of oxide will, on its becoming cold, 

 be so thick as to scale off. 



If the polished metal be oiled, previously to its being heated, 

 no change in its colour will be produced ; since the oil defends 

 it from the action of the atmosphere. Analagous changes of 

 colour may also be produced on the bright surface of copper, by 

 heat, in consequence of oxide on its surface. This colour instant- 

 ly disappears, and the surface becomes bright, by contact of resin 

 or grease of any kind; since these substances re-convert the 

 oxide into metal. 



" III. — What law of mechanics will account for the superior 

 power of a long screw-driver, though the handle be no larger than 

 that of a short one ? '' 



It is an indisputable fact that a screw can be driven " home '* 

 much more easily and forcibly by a long screw driver than by a 

 short one, even when the handles of both are similar. Many 

 have supposed that this arises from an actual increase of mechani- 

 cal power, which, they have attempted to show, results from the 

 application of the moving power at a greater distance from the 

 body to be moved. But this principle of mechanics will not be 

 found at all applicable to a case of this kind, as will be presently 

 shown ; and the only advantage gained, is the greater facility of 

 applying and using any given power. In order to set this matter 

 in the fairest point of view, the following experiments were m«idc 



