SCIENCE OF COMMON THINGS. 39 



Mortar. Htucco. Color of bricks. 



and unites them together. The lime also gradually 

 becomes converted into carbonate of lime. 



336 What sand is mest suitable for the formation of mortar f 



That which is wholly silicious and is sharp / that is, 

 not having its particles rounded by attrition. 



337* What are the proportions of lime and sand in good mortar f 



The proportions are varied in different places : the 

 amount of sand, however, always exceeds that of the 

 lime. The more sand that can be incorporated with 

 the lime the better, provided the necessary degree of 

 plasticity is preserved. 



338 What are water, hydraulic, or Roman cements f 



Those which have the property of hardening under 

 water, and of consolidating almost immediately on be- 

 ing mixed. 



330 To what cause do the water-cements owe their property of becoming 

 hard under water f 



The cause is not satisfactorily known : all water- 

 cements contain a portion of burnt clay, which proba- 

 bly absorbs immediately all superabundant moisture 

 from the lime, and thus expedites its solidification. 



This explanation is rendered more probable from the fact, that if the 

 clay is burnt sufficient to vitrify it or convert it into brick, it ceases to 

 form a water-cement. 



330 What are the constituents of a water-cement f 



Quicklime, sand or silica, and a proportion of clay. 



331 What is stucco f 



Stucco is composed of various ingredients, generally 

 of "plaster of Paris," sometimes ot white marble pul- 

 verized and mixed with plaster and lime. 



333 What is terra-cotta f 



Literally, baked clay, a name given to statues, archi- 

 tectural ornaments, vases, figures, etc., modelled of 

 potters'-clay and fine colorless sand, and afterwards 

 exposed to a most intense heat. 



333 Why are bricks when burned usually of a red color ? 



Because the iron contained in the clay is converted 



