CEMENTS. 51 



more quickly, when the lime and water are less in 

 quantity and more subdivided. From two to four parts 

 of sand are used to one of lime, according to the quality 

 of the lime, and the labor bestowed on it. The more 

 pure is the lime and the more thoroughly it is beaten, or 

 worked over, the more sand it will take up, and the 

 more firm and durable does it become. 



Clay. This abundant and useful earth is composed 

 principally of alumine and silex. It possesses the 

 valuable property of forming, when wet, a ductile and 

 tenacious paste, which is changed by heat to a stony 

 hardness. Common clay, of which bricks and coarse 

 potter's ware are made, contains oxide of iron, which 

 causes it to turn red in burning. The purer sorts, such 

 as pipe-clay, become whiter when exposed to a high 

 heat. The earthy smell which clays emit when 

 breathed upon appears also to be owing to oxide of 

 iron. Absolutely pure clay emits no smell. Refractory 

 clays are those which endure the greatest heat without 

 melting. The best fire-proof bricks and crucibles are 

 made from slate clay, and contain a good deal of sand. 

 Sometimes they are made of old materials, which have 

 been before exposed to high heat, pounded up and 

 mixed with fresh clay. A mixture of two parts of 

 Stourbridge clay, and one part of coke, has been found 

 very refractory. 



Ashestus. Asbestus is a mineral of a fibrous struc- 

 ture. One of its varieties, called amianthus, is com- 

 posed of very delicate, flexible filaments, resembling 

 fibres of silks. It has been manufactured into cloth 

 and paper, which possess the property of being incom- 

 bustible. It is difficult; however, to find fibres of suffi- 

 cient length and firmness to produce objects of any 

 great use. It is sometimes mixed with clay, in pottery, 

 to increase its strength. It has also been used for the 

 packing of steam-engines which are of high pressure, 

 or in which steam is used at an elevated temperature. 



