BOOK XII. 



585 



A — Mouth of the tunnel 



Little trough. 



the others, for which reason crystals take the first place. From these, when 

 pounded, the most excellent transparent glass was made in India, with which 

 no other could be compared, as Pliny relates. The second place is accorded 

 to stones which, although not so hard as crystal, are yet just as white and 

 transparent. The third is given to white stones, which are not transparent. 

 It is necessary, however, first of all to heat all these, and afterward they are 

 subjected to the pestle in order to break and crush them into coarse sand, 

 and then they are passed through a sieve. If this kind of coarse or fine sand 

 is found by the glass-makers near the mouth of a river, it saves them much 

 labour in burning and crushing. As regards the solidified juices, the first 

 place is given to soda ; the second to white and translucent rock-salt ; the third 

 to salts which are made from lye, from the ashes of the musk ivy, or from 

 other salty herbs. Yet there are some who give to this latter, and not to the 

 former, the second place. One part of coarse or fine sand made from fusible 

 stones should be mixed with two parts of soda or of rock-salt or of herb 

 salts, to which cure added minute particles of magnes}^ It is true that in our 



^*The statement in Pliny (xxxvi., 66) to which Agricola refers is as follows : " Then 

 " as ingenuity was not content with the mixing of nitrum, they began the addition of lapis 



