LIMESTONES. 367 



varieties in the United States contain 20 to 40 per cent, of 

 magnesia, and 12 to 30 per cent, of silica and alumina, A 

 variety worked extensively at Rondout, N. Y., afforded Prof. 

 Beck, carbonic acid 34*20, lime 25*50, magnesia 12*35, silica 

 15*37, alumina 9*13, peroxyd of iron 2*25.* Oxyd of iron 

 is rather prejudicial than otherwise. 



In making mortar, the lime is mixed with water and 

 siliceous sand. The final strength of the mortar depends 

 principally on the formation of a compound between water, 

 the silica (or sand) and the lime ; of course therefore the 

 finer the sand, the more thorough the combination. In 

 hydraulic lime, there is silica and alumina present in a thor- 

 oughly disseminated and finely divided state, which is favor- 

 able for the combination alluded to ; and to this fact appears 

 to be mainly owing its hydraulic character. Much less sand 

 is added in making mortar from this lime than from that of 

 ordinary limestone. 



Pozzuolana (page 347) forms a hydraulic cement when 

 mixed with a little lime and water. Similar cements may be 

 made with tufa, pumice stone, and slate clay, by varying the 

 proportions of lime ; these materials consist essentially of 

 silica and alumina or magnesia with alkalies, and often some 

 lime, and therefore produce the same result as with hydrau- 

 lic limestone. 



In the burning of lime, the most common mode is to erect 

 a square or circular furnace of stone, with a door for manag- 

 ing the fire below. An arched cavity for the fire is first 

 made of large pieces of limestone, and then the furnace 

 is filled with the stone placed loosely so as to admit of the 

 passage of the flame throughout : the carbonic acid is ex- 

 pelled by the heat, and when the fires are out, the lime now 

 in the state of quicklime, or in other words, pure lime, is 

 taken out. Great economy of fuel is secured by means of 

 what is called a perpetual kiln. The cavity within is best 

 made nearly of the shape of an egg with the narrow end 

 uppermost. The inner walls are of quartz rock, mica slate, 

 or some refractory stone or fire brick, and between the inner 

 and outer there is a layer of cinders or ashes, as in the iron 

 furnace, page 233. Below are three or more openings for 

 furnaces which lead into the main cavity, a few feet from the 

 bottom ; and alternate with these are other openings at a 



Mineralogy of New York, page 78. 



