34 Laboratory Arts 



possibilities of burnishing to a brilliant reflecting surface, or of 

 frosting to a beautiful grey finish, give a range for the exercise 

 of individual taste not possessed by many metals. Its conduc- 

 tivity is the highest known, and its rate of expansion is accur- 

 ately known, in common with most of its physical constants, 

 Silver is used as a standard of comparison largely because of 

 the purity in which it can be obtained, the high melting point, 

 and the non-corroding nature of the metal. 



Lead is a metal that should surely be more used in the 

 making of apparatus. It corrodes very slowly, it is easily beaten 

 into any desired shape, it may be cast in the roughest of moulds, 

 and makes excellent joints between bolted surfaces. It is used 

 for making heavy bases for instruments that would otherwise be 

 " top heavy," for making small crucibles for use with hydro- 

 fluoric acid and for the entire drainage systems of physical and 

 chemical laboratories ; though, owing to the action of mercury 

 upon it, it is now being replaced by enamelled earthenware for 

 this purpose. 



The ease with which it is beaten or hammered into any 

 desired shape makes it of great service in a laboratory. Lead 

 wire is a case in point. This wire is now frequently placed 

 between two flanges of a joint, and when these flanges are 

 bolted tightly together, the lead is squeezed into practically 

 perfect contact with each flange, and a secure hydraulic joint 

 results. Similarly in the adjustment of instruments put together 

 with studs or nuts, a piece of lead wire is sometimes passed 

 round outside the studs, so that in pulling up the two surfaces 

 one nut may be tightened more than another, and the axes of 

 the two portions brought exactly into line. 



Lead wire is also useful for the manufacture of "riders" 

 in lever experiments, numbers 16 to 20 being the most 

 suitable. 



II. TOOLS. 



The tools for metalwork are of a different order from 

 woodworking tools, since we have to deal with a much harder 

 material. Metalwork is therefore more difficult than woodwork, 



