2. That which is found in Pontus and Gaul is black, because those
countries are nearest to the north. As one passes on from north to west,
it is found of a bluish shade. Due east and west, what is found is of a
violet shade. That which is obtained in southern countries is naturally
red in quality, and therefore this is found in the island of Rhodes and
in other such countries that are nearest to the course of the sun.

3. After the shellfish have been gathered, they are broken up with iron
tools, the blows of which drive out the purple fluid like a flood of
tears, and then it is prepared by braying it in mortars. It is called
"ostrum" because it is taken from the shells of marine shellfish. On
account of its saltness, it soon dries up unless it has honey poured
over it.

CHAPTER XIV

SUBSTITUTES FOR PURPLE, YELLOW OCHRE, MALACHITE GREEN, AND INDIGO

1. Purple colours are also manufactured by dyeing chalk with madder root
and with hysginum. Other colours are made from flowers. Thus, when
fresco painters wish to imitate Attic yellow ochre, they put dried
violets into a vessel of water, and heat them over a fire; then, when
the mixture is ready, they pour it onto a linen cloth, and squeeze it
out with the hands, catching the water which is now coloured by the
violets, in a mortar. Into this they pour chalk and bray it, obtaining
the colour of Attic yellow ochre.

2. They make a fine purple colour by treating bilberry in the same way
and mixing it with milk. Those who cannot use malachite green on account
of its dearness, dye blue with the plant called dyer's weed, and thus
obtain a most vivid green. This is called dyer's malachite green. Again,
for want of indigo, they dye Selinusian or anularian chalk with woad,
which the Greeks call [Greek: isatis], and make an imitation of indigo.

3. In this book I have written down, so far as I could recall them, the
methods and means of attaining durability in polished finishings, how
pictures that are appropriate should be made, and also the natural
qualities of all the colours. And so, having prescribed in seven books
the suitable principles which should govern the construction of all
kinds of buildings, I shall treat in the next of water, showing how it
may be found in places where it is wanting, by what method it may be
conducted, and by what means its wholesomeness and fitness may be
tested.

BOOK VIII

INTRODUCTION

1. Among the Seven Sages, Tha