THE BOULEVARDS. 131 



or natural bitumen. It may be asked bow it is tbat ordi- 

 nary tar or pitcb of good quality mixed with pounded lime- 

 stone does not answer the purpose of this natural combina- 

 tion ; but it is found by experiment that, although natural 

 bitumen differs but slightly in chemical composition from 

 pitch and tar, it is much more elastic and durable. If made 

 with tar, the resulting asphalte is sticky and soft in hot 

 weather ; if with pitch, it is too brittle, and soon cracks and 

 spbts. 



In the natural asphaltic rock the bitumen is so intimately 

 combined with the calcareous matter, that it not only resists 

 the action of the air and water for a considerable time, but 

 even that of some of the strong mineral acids. The ancients 

 were in the constant habit of using natural bitumen instead 

 of mortar ; and there is a tradition that the stones of the 

 Tower of Babel were cemented together with the same ma- 

 terial as that forming the footways of the boulevards. The 

 principal ingredients used in forming the mastic for the 

 pavement is the dark brown bituminous limestone from 

 Pyrimont, just described. The stone is first reduced to fine 

 powder, and then mixed with a certain proportion of mineral 

 bitumen, extracted previously from another portion of it. 



When it is intended to be used for covering roofs, lining 

 tanks, &c, no other addition is necessary ; but if it is to be 

 used for paving, a certain quantity of sea-grit is added. 

 One specimen analysed by an English chemist yielded 29 of 

 bitumen, 52 of limestone, and 19 of siliceous sand. The 

 ingredients are exposed for some hours to a strong heat in 

 large cauldrons, and kept constantly stirred by machinery. 

 The mastic thus formed is made into blocks, measuring 

 eighteen inches square by six inches deep, and weighing 

 from 1121b. to 1301b. each. In this state they are sold 

 ready for use, and are remelted on the spot where the 

 asphalte has to be applied ; for which purpose small portable 

 furnaces fitted with cauldrons are employed. A pound 

 weight of mineral bitumen is first put into the cauldron, and 

 when melted 561b. of the mastic are added, the whole being 

 repeatedly stirred. When fully mixed, another 561b. of 

 mastic are stirred in, and so on until the cauldron is full. 



k2 



