ART OF BUILDING. 239 



(day), magnesia, and the oxides of iron (iron rust 4*c, 

 sometimes with sulphate of lime (lime and oil of vitriol 

 called gypsum, or plaster of Paris) when the limestone is 

 hard enough to scratch glass it contains quartz, when of 

 a brown or red color it contains oxide of iron, when it 

 effervesces slowly, producing a milky appearance it con- 

 tains magnesia ; and when black with a bad smell it con- 

 tains coaly substances. Magnesia and clayey substances 

 impair the quality of the quicklime by preventing its 

 setting wtU. 



The hardest limestones make the best quicklime. 



There are two kinds of quicklime used by masons, 

 common lime and hydraulic or water lime ; the latter dif- 

 fers from the former in containing iron and manganese. 

 The best common lime comes from Thomastown, Me. 

 A good hydraulic lime (cement) manufactured in New 

 York, was used for constructing the locks of the Grand 

 Canal. 



Lime is nature's universal cement, and is employed 

 frequently in an immense number of her combinations. 

 Besides the great masses of limestone on mountains as 

 well as in plains, besides the great variety of compound 

 stony substances, widely diversified, of which it forms 

 an essential part, it is found in vegetables. The bones 

 and shells of all animals are formed of the same substance 

 united with certain acids. 



Sand is the next important ingredient of mortar. On 

 the quality of this, essentially depends the quality of the 

 mortar, and if it contains any clay or mud, or is brought 

 from the shores of salt waters, it is unfit lor mortar 

 until it is washed, because the clay or salt will prevent 

 its setting, these substances having a greater affinity 

 for water than for carbonic acid. The sharper and 

 coarser the grain of the sand, the better the mortar 

 and the less lime is required, which of course diminishes 

 the price ; it should be a general rule to use no more lime 

 than what is just necessary to cover every particle of the 

 sand. 



The celebrated Rondelet, a French engineer, and Mr 

 Smeaton, the builder of the Eddystone light-house, made 

 numerous experiments on mortar. The following are 

 some of their conclusions. 



