BOOK XII. 585

A—Mouth of the tunnel. B— Trough. C— Tanks. D —Little trough.

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the others, for which reason crystals take the first place. From these, whenpounded, the most excellent transparent glass was made in India, with whichno other could be compared, as Pliny relates. The second place is accordedto stones which, although not so hard as crystal, are yet just as white andtransparent. 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 aresubjected 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 sandis found by the glass-makers near the mouth of a river, it saves them muchlabour in burning and crushing. As regards the solidified juices, the firstplace is given to soda ; the second to white and translucent rock-salt; the thirdto salts which are made from lye, from the ashes of the musk ivy, or fromother salty herbs. Yet there are some who give to this latter, and not to theformer, the second place. One part of coarse or fine sand made from fusiblestones should be mixed with two parts of soda or of rock-salt or of herbsalts, to which are added minute particles of magnes . 16 It is true that in our

16 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