For a moment, however, I must touch on the subject of billiard balls. It may interest our readers to know that the size of the little black dot on a ball indicates its quality. The nerve which runs through a tusk, is visible at this point, and a ball made from the ivory near the end of the tusk, where the nerve has tapered off to its smallest proportions, is the best ball. The finest balls of all are made from short stubby tusks, which are known as "ball teeth." The ivory in these is closer in grain, and they are much more expensive. Very large tusks are more liable to have coarse grained bony spaces near the centre. CHAPTER X INLAY AND MOSAIC There are three kinds of inlay, one where the pattern is incised, and a plastic filling pressed in, and allowed to harden, on the principle of a niello; another, where both the piece to be set in and the background are cut out separately; and a third, where a number of small bits are fitted together as in a mosaic. The pavement in Siena is an example of the first process. The second process is often accomplished with a fine saw, like what is popularly known as a jig saw, cutting the same pattern in light and dark wood, one layer over another; the dark can then be set into the light, and the light in the dark without more than one cutting for both. The mosaic of small pieces can be seen in any of the Southern churches, and, indeed, now in nearly every country. It was the chief wall treatment of the middle ages. [Illustration: MARBLE INLAY FROM LUCCA] About the year 764, Maestro Giudetto ornamented the delightful Church of St. Michele at Lucca. This work, or at least the best of it, is a procession of various little partly heraldic and partly grotesque animals, inlaid with white marble on a ground of green serpentine. They are full of the best expression of mediaeval art. The Lion of Florence, the Hare of Pisa, the Stork of Perugia, the Dragon of Pistoja, are all to be seen in these simple mosaics, if one chooses to consider them as such, hardly more than white silhouettes, and yet full of life and vigour. The effect is that of a vast piece of lace,--the real cut work of the period. Absurd little trees, as space fillers, are set in the green and white marble. Every reader will remember how Ruskin was enthusiastic over these little creatures, and no one can fail to feel their charm. The pavements at the Florentine Baptistery and at San Miniato are interesting examples of inlay in black and white marble. They are early works, and are the natural forerunners of the marvellous pavement at Siena, which is the most remarkable of its kind in the world.