Page:Popular Science Monthly Volume 45.djvu/332

 through fifty-two natural orders of plants. Sixteen of these are heavier than water, and have a specific gravity varying from 1·30.20 in the black iron wood of southern Florida to a white oak (Quercus grisea) of New Mexico with the wood only slightly heavier than water—namely, 1·0092 for its specific gravity. It is interesting to note in passing that all these sixteen kinds of wood that will sink in water are natives of southern Florida, a semitropical region, and the South and West regions, none of them growing in the Mississippi Valley or east of it.

The black ironwood above mentioned as having the heaviest wood is in many respects a striking contrast with the giant redwood (Sequoia) of California, which is not only the largest of our trees, but its wood is among the lightest, it having a specific gravity of only 0·2884, or about one fourth as heavy as the ironwood,



which latter is a small, gnarly tree of no value as building timber.

It was said that there are four hundred and twelve species of timber receiving treatment in the census report, and therefore it is appropriate to show the peculiarities of the one that stands midway of this long list as regards its specific gravity, and especially so as it is one of the more common sorts and a very valuable timber for many purposes—namely, the ash (Fraxinus).

Fig. 3 shows the appearance of this wood as seen looking upon the smooth surface of the end of a stick of timber. It is a decidedly porous wood, as indicated by the minute, light dots which are arranged in a series of curved belts in the engraving.

This leads us naturally to consider somewhat in detail the general make-up of a stem or trunk of a tree. The primary division of the parts is into the wood and the bark. The latter is