4 SCIENTIFIC AMUSEMENTS. 



in the latter case, the wax is first heated so as to render it plastic 

 enough to take an impression, and that much less pressure is 

 required to produce the device. 



Some soft solids, like putty and day, are naturally plastic 

 without heating, so that by the pressure of the hands alone they 

 can be moulded into all sorts of shapes ; good clay of proper con- 

 sistency, when thus moulded into form, becomes dried and 

 hardened, or indurated, by exposure to air, and especially by 

 subsequent heating by fire ; the art of pottery, founded on this 

 property, is one dating from the very earliest periods, and one of 

 the most important manufactures that have descended to us from 

 prehistoric times. Little but patience and some degree of manual 

 dexterity is requisite to enable any one in the possession of a 

 lump of properly tempered clay to exercise this art in a rough 

 way, and to fashion such articles as pots and saucers, or even to 

 mould and model more artistic and ornamental objects ; although 

 the after processes of firing, glazing, and the like, usually requisite 

 to enable the objects thus fashioned to become permanent, often 

 require a considerable degree of skill and care to carry them out 

 properly. 



Expt. 1. To mould Crucibles out of Clay. Clay pots so 

 prepared as to stand the fire without cracking are largely employed 

 in the arts for the purpose of melting substances of various 

 kinds, such as metals and glass. To prepare such crucibles of 

 the best kind, a particular sort of clay known as "fire-clay " is 

 necessary ; but small crucibles of sufficient firmness for some ex- 

 periments can often be made from a good sample of tenacious 

 brick clay free from stones and well " tempered " by continual 

 rolling-out between the hands and kneading together ; finally the 

 mass is worked into the shape of a small plant-pot (without a hole 

 in the bottom) or thick cup, and set aside to dry in the air. 

 After some days the drying may be completed before a fire or in 

 the oven, when (if the clay is sufficiently good) the crucibles can 

 be heated red hot in the fire without cracking, and may be used 

 for melting lead, and such like operations requisite in certain kinds 

 of experiments. 



Crucibles of various materials and shapes can be cheaply pur- 

 chased at the instrument dealers, so that except for the amuse 

 ment of making one's own apparatus it is not absolutely necessary 

 to prepare them oneself. Thin porcelain crucibles (fig. 1) are used 

 for some kinds of laboratory operations ; for others, thicker and 

 coarser ones of clay mixed with sand or powdered plumbago 

 (blacklead) to enable them to stand heat better (fig. 2). In order 

 to lift hot crucibles, specially shaped metal tongs (fig. 3) are con- 



