128 Laboratory Arts 



contact, air-bubbles, etc., pour into the still warm mould some 

 melted vaseline, and remove any not absorbed by the porous 

 surface. On no account should this be attempted while the 

 mould is damp. 



Proceed then to fill the mould with a mixture of plaster of 

 Paris and water prepared exactly as before, smoothing the top 

 with a spatula, to form the base of the new pyramid. In a few 

 hours the pyramid may be removed by inverting the mould, and 

 shaking, or by gently inserting the blade of a spatula between 

 mould and cast. 



Five similar pyramids may then be similarly cast, and the 

 cube completed. 



The faces of these pyramids will probably be somewhat 

 stained, but a few strokes upon a sheet of fine glass-paper (not 

 with the glass-paper) will give them a clean, white surface, 

 provided they are perfectly dry before this is attempted. 

 Models such as this, which have to withstand fairly rough 

 usage, are greatly improved by a soaking in boiling paraffin 

 wax, removing while still hot, immediately after effervescence 

 has ceased. 



The process outlined above may be varied in order to 

 produce moulds of spheres, coins, medals, and other objects, 

 used for casting lead, plaster, sulphur (which produces excel- 

 lent casts in warmed moulds) and other substances, useful in 

 numerous ways in the laboratory. Sulphur, being an excellent 

 electrical insulator, may be by these means made up into 

 extremely useful units of electrical demonstration apparatus, 

 and in any required form. Moulds for casting sulphur should 

 not be vaselined, as the cooled sulphur comes away quite 

 easily, provided the temperature at casting has been below the 

 viscous point; though it will have been necessary to heat 

 above this temperature in order to secure uniformity of fusion. 



Lead may be cast in warm, dry plaster moulds, being 

 melted for the purpose in a porcelain dish over a Bunsen 

 flame. No injury will be done to the dish if dry. 



