BOOK V	111
	an equal quantity of rock salt is added to the sand and if there is no rock salt, marine salt can be used. When none of these salts is available either artificial salts or salts obtained from leaching the ashes of anthyllis and some even add the ashes of burnt trees. According to Pliny the first glass was made in India from crushed quartz. It can also be made from a genus of small pebbles very similar to gem stones that melt in a fire in the same fashion as the materials mentioned above. At one time Sidon was famous for the glass produced there but within our times the finest glass is made in Murano and made famous by the Venetians who live in a city renowned for its beauty and spaciousness. The glass which is as colorless and transparent as quartz is the most highly esteemed. The clearest and finest glass is tinted in two ways. Sometimes a small or large amount of natural coloring material is ground with the glass and then both are melted together and it is only in this way that glass with the true color of gems is produced, for example, diamond, srnaragdus, carbunculus, amethyst, hyacinthus, sapphire, jet and others having a single color as well as some of the multicolored gems such as opal. By another method an apparently black glass is produced which, if held to the sun will show the true color that this glass will give to another glass when used as a dye. Silver is used to color glass white, black, green or part blue and part purple. In the same manner a famous variety of dyeing glass is made from gold and this is used to tint glass a clear ruby red. Black glass is called obsidianus because of obsidanus lapis which is also called jet. According to Pliny all red glass which is not transparent is called haematinon. The small jugs from which we drink malt liquors and the vessels from which we eat belong to this class. But this is enough con­cerning this and the stones that melt in fire. I shall now take up the gems.