Wood contains (1) water, (2) ligno-cellulose, (3) extractives, (4) ash or mineral matter. The water, in thoroughly-seasoned and air-dried wood, generally constitutes about one-eighth part of its weight. The main constituent of wood is the so-called ligno-cellulose, which is present to the extent of from 75 to 85 per cent. It may be resolved into two substances, which, for convenience' sake, are here called cellulose and lignose. The extractives belong to two groups — one, soluble in alcohol and ether, consists chiefly of resins ; the other, soluble in cold or hot water, or else in very dilute alkalies, includes tannin, albuminoids, gum, and colouring-matters. The following analyses of three kinds of wood in an air-dried state will convey a fair idea of their constitution in 100 parts:
| Mahogany | Oak | Pine | |
| Water | 12.4 | 13.1 | 12.9 |
| Cellulose | 49.0 | 39.5 | 53.3 |
| Lignose | 27.6 | 34.3 | 28.2 |
| Ash | 1.1 | 1.2 | 0.3 |
| Resin | 1.0 | 0.9 | 1.6 |
| Water-extract | 8.9 | 11.0 | 3.7 |
The preparation of panels for painting requires much time and trouble. The directions given by ancient authorities are numerous, and not always accordant. One author tells us to boil the wood ; another says we are to coat it with mastic dissolved in twice-distilled turpentine and mixed with white. Then it is to be treated twice or thrice with spirits of wine, in which some white arsenic or corrosive sublimate has been dissolved ; coats of boiled oil, of liquid-varnish[1] and white, and of verdigris and yellow are subsequently mentioned. Probably the best method of treating the harder woods intended for pictures is, after
- ↑ Made by boiling i part of sandarac in 3 parts of linseed-oil.
