MORTAR-MAKING. 897 



The origin of dolomite has loii<;' been a standing enigma to 

 the geologist ; but as the question is one of only theoretical 

 interest, its discussion would be out of place in this paper. 



The dolomites of Durham are employed iu the chemical 

 works on the Tyne for the manufacture of sulj)hate of 

 magnesia (lilpsom salts), and is greatly used for the manu- 

 facture of carbonate of magnesia. 



Mr. Rudler says that if this stone is properly calcined it 

 will furnish an excellent hydraulic lime, and quotes M. H. 

 St. Claire Deville, who has called attention to the powerful 

 hydraulic properties of caustic magnesia by its union with 

 water to form a definite hydrate, and Mr. Rudler also 

 mentions that if the stone is calcined below a I'ed heat, the 

 carbonate of magnesia suffers decomposition, whilst the 

 carbonate of lime remains almost intact ; on slaking this 

 calcined dolomite the magnesia combines with water, and 

 this hydrate cements the unaltered particles of carbonate of 

 lime. 



Mr. Vicat, a well known writer on the properties of limes, 

 classifies them somewhat as follows : — 



" Fat, or common lime, which gains no consistency under 

 water, remaining in a state of paste in water unchanged, but 

 dissolving wholly in pure water frequently changed. 



" Poor lime, which is a combination of lime and sand, the 

 lime in which exhibits the same phenomena as if no sand 

 were present, slightly hydraulic limes obtained from limestone 

 containing 8 to 12 per cent, of other substances such as silica, 

 magnesia, iron, &c. ; these set in about 20 days after 

 immersion, but in a year have not gained a consistency greater 

 than hard soap. They dissolve in pure water, but very slowly. 



" Hydraulic limes from limestone containing from 12 to 20 

 per cent, of the above-mentioned ingredients ; these set in 

 from six to eight days, and in six months acquire the hardness 

 of soft stone. 



" Extra hydraulic limes from limestones containing 20 to 30 

 per cent, of the same ingredients ; they set in from two to 

 four days, and have attained great hardness in a single month. 

 In six months they resemble the hard calcareous stones which 

 bear cutting." 



With regard to the manipulation of limes in making mortar 

 it seems to matter little whether pure lime is slaked in large 

 or small quantities at once ; but with hydraulic limes only so 

 much should be slaked out at a time as can be worked otjf 

 within the next eight or ten days, 



