22 SCIENTIFIC AMUSEMENTS. 



vessel In some cases, before complete liquefaction is brought 

 about, the solid passes through an intermediate stage, becoming 

 pasty ; but this is not the general rule. Conversely, most liquids 

 when sufficiently cooled down lose their liquidity and become 

 solid masses, as water does when it freezes to ice. Some liquids, 

 such as quicksilver (otherwise termed mercury), require the cold of 

 the Arctic Kegions to make them freeze, and some fe^, such as 

 alcohol (spirits of wine), do not thoroughly solidify until the most 

 intense degrees of cold possible are attained. 



In the same sort of way some solids require a considerable amount 

 of heat to be applied before fusion or melting happens ; whilst others 

 melt so readily that they have to be cooled down artificially in 

 order to keep them solid. Thus a block of ice melts to liquid 

 water on the application of but an inconsiderable amount of heat, 

 comparatively speaking, whilst a mass of lead or zinc must be 

 heated nearly red hot before it melts, and platinum requires the 

 very highest temperature attainable in special furnaces. 



Many useful arts depend on the kind of operation known as 

 " casting," based on the property of substances to become liquid 

 when heated and to- solidify or freeze again on cooling. This is 

 carried out very much in the same way as in the experiment above 

 described for making a wax lemon (Expt. 5). A mould of suitable 

 material is prepared, hollow or otherwise according to the shape of 

 the object to be cast, and the melted matter to be cast poured into 

 the mould until it is filled. After cooling the solidified matter is 

 extracted from the mould, and dressed down and cleansed and 

 polished on the surface if required. The large gas and water pipes 

 laid down in the streets are thus cast of molten iron ; brass handles, 

 door knockers, and similar objects are frequently similarly prepared 

 from melted brass ; and plate-glass windows are made by melting 

 glass and then pouring it out on a flat metal table so that it solid- 

 ifies in a horizontal sheet on cooling ; in fact, innumerable objects 

 and articles of everyday use are fashioned by means of this casting 

 process. Rough iron objects are generally cast in moulds of sand, 

 not wet but slightly damped so as just to adhere together. The 

 heat required to melt cast iron is too great to enable this substance 

 to be easily experimented with ; but lead can be readily employed 

 for the purpose. 



Expt. 15. To cast Lead into Sticks. Get some sand in a pot or 

 box and damp it very slightly, well intermixing it : the quantity 

 of water used should be the smallest amount possible, only just 

 sufficient to prevent the sand from falling in and filling up the 

 hole left when a pencil is rammed down into the sand (previously 

 gently pressed down in the box) and then cautiously withdrawn ; 



