Rapid drying of wood tends to "case harden" it, _i.e._, to dry and shrink the outer part before the inside has had a chance to do the same. This results in checking separately both the outside and the inside, hence special precautions need to be taken in the seasoning of wood to prevent this. When wood is once thoroly bent out of shape in shrinking, it is very difficult to straighten it again. Woods vary considerably in the amounts of their shrinkage. The conifers with their regular structure shrink less and shrink more evenly than the broad-leaved woods.[3] Wood, even after it has been well seasoned, is subject to frequent changes in volume due to the varying amount of moisture in the atmosphere. This involves constant care in handling it and wisdom in its use. These matters are considered in _Handwork in Wood_, Chapter III, on the Seasoning of Wood. THE WEIGHT OF WOOD. Wood substance itself is heavier than water, as can readily be proved by immersing a very thin cross-section of pine in water. Since the cells are cut across, the water readily enters the cavities, and the wood being heavier than the water, sinks. In fact, it is the air enclosed in the cell cavities that ordinarily keeps wood afloat, just as it does a corked empty bottle, altho glass is heavier than water. A longitudinal shaving of pine will float longer than a cross shaving for the simple reason that it takes longer for the water to penetrate the cells, and a good sized white pine log would be years in getting water-soaked enough to sink. As long as a majority of the cells are filled with air it would float. In any given piece of wood, then, the weight is determined by two factors, the amount of wood substance and the amount of water contained therein. The amount of wood substance is constant, but the amount of water contained is variable, and hence the weight varies accordingly. Moreover, considering the wood substance alone, the weight of wood substance of different kinds of wood is about the same; namely, 1.6 times as heavy as water, whether it is oak or pine, ebony or poplar. The reason why a given bulk of some woods is lighter than an equal bulk of others, is because there are more thin-walled and air-filled cells in the light woods. Many hard woods, as lignum vitae, are so heavy that they will not float at all. This is because the wall of the wood cells is very thick, and the lumina are small. In order, then, to find out the comparative weights of different woods, that is, to see how much wood substance there is in a given volume of any wood, it is necessary to test absolutely dry specimens. The weight of wood is indicated either as the weight per cubic foot or as specific gravity. It is an interesting fact that different parts of the same tree have different weights, the wood at the base of the tree weighing more than that higher up, and the wood midway between the pith and bark weighing more than either the center or the outside.[4]