v.] THE LIME IN THE MORTAR. 113 



earlier Egyptian buildings, though the stones are ex- 

 quisitely squared and polished, are put together like- 

 wise without mortar. So, long ages after, were the 

 earlier Eoman buildings, and even some of the later. 

 The famous aqueduct of the Pont du Gard, near 

 Nismes, in the south of France, has, if I recollect 

 right, no mortar whatever in it. The stones of its 

 noble double tier of circular arches have been dropped 

 into their places upon the wooden centres, and stand un- 

 moved to this day, simply by the jamming of their own 

 weight; a miracle of art. But the fact is puzzling; 

 for these Eomans were the best mortar makers of the 

 world. We cannot, I believe, surpass them in the art 

 even now ; and in some of their old castles, the mortar 

 is actually to this day harder and tougher than the 

 stones which it holds together. And they had plenty 

 of lime at hand if they had chosen to make mortar. 

 The Pont du Gard crosses a limestone ravine, and is 

 itself built of limestone. But I presume the cunning 

 llomans would not trust mortar made from that coarse 

 Nummulite limestone, filled with gritty sand, and pre- 

 ferred, with their usual carefulness, no mortar at all to 

 bad. 



But I must return, and tell my readers, in a few 

 words, the chemical history of mortar. If limestone be 

 burnt, or rather roasted, in a kiln, the carbonic acid is 

 given off as you may discover by your own nose ; as 

 many a poor tramp has discovered too late, when, on 

 a cold winter night, he has lain down by the side of 

 the burning kiln to keep himself warm, and woke in 

 the other world, stifled to death by the poisonous 

 fumes. 



The lime then gives off its carbonic acid, and also 



