Popular Science Monthly/Volume 45/July 1894/Sunshine Through the Woods

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THE title above might suggest a forest that has been shot through by the light of day, or some delightful dell where the rays of the sun make every spot enchanting. Quite otherwise, the lines to follow deal with the printing of pictures of sections of woods by means of the direct sunlight, and some of the points of structure thus brought to view.

If any object through which the light passes unequally in its various parts be brought close against a sensitized paper used by photographers in printing pictures from their negatives, it is evident that an impression will be produced. This print will be a negative, or, in other words, the dark parts in the subject will be light and the light parts dark. For example, the section of papaw wood shown in Fig. 1 is a negative, while in Fig. 3 is the positive, and corresponds closely with the wood itself in its light and dark parts.

The first essential in getting prints of woods is to obtain uniformly thin sections of the wood. These are not far to seek, for Mr. R. B. Hough has become famous for his wood sections. The

Fig. 1.—Section of Papaw Wood. Negative.

process by which he is able to obtain his beautiful sections is not known to the writer; but to him thanks are due for the specimens which have been used in making the prints to illustrate this paper. Having glanced at the two mentioned engravings and remembering that very much of the fineness of detail is necessarily lost in the engraving process, the reader is ready to consider the method of making the prints. The sections of wood having been secured, the only other things needful are a few "printing frames" (one will answer) of the ordinary sort used by photographers. Instead of the glass negative which the photographer uses and has prepared in the dark room, a simple plain pane of glass is needed. This is placed in the frame; upon it is put the section of wood, and over the latter a sheet of the sensitized paper. This paper is brought close upon the wood by means of the clamps, and the frame is ready to be placed in the sunlight. After the print is made, which takes only a few minutes, the time depending upon the strength of the light and the porosity and translucency of the wood, the print is subjected to the toning process, and, after washing and drying, is ready to become the negative from which the final print is made. In order that the light may pass more readily through the negative it is soaked for a few minutes in kerosene and wiped dry upon the surface. The negative is then placed paper side down upon the plain glass in the printing frame, and upon its face is brought the sensitive side of a fresh sheet of paper, the two sheets being pressed close to each other and evenly against the glass by the clamps, as before stated. In a very brief period a positive print is obtained, which upon removal is toned in the usual way, and becomes a picture the one, for example, furnishing the subject for the engraving in Fig. 2.

What with the brief description of the manner in which solar prints of translucent objects are made, the reader may wish to go

Fig. 2.—Section of Papaw Wood. Positive.

further and consider some of the differences of detail in the various kinds of wood, for one kind of timber differs from another in many ways. Should we, for example, turn to the Report on the Forests of North America, in the last census, no less than four hundred and twelve kinds of timber would be found distributed 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,

Fig. 3.—Cross Section of Ash Wood.

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 shown in Figs. 1 and 2 as a substance quite different from the wood that lies within, and is protected by it. Growth of the stem of ordinary trees takes place in a continuous zone just beneath the bark, the latter being also supplied with new material, as it may be needed to supply the same formative layer. As the years roll on, the wood first made, while the stem was small, and

Fig. 4.—Cross Section of Pin Oak. Positive.

now situated near the center, changes its appearance by taking on some color, the shade being determined by the kind of wood. In some of the "precious woods," so called because of their great value for special purposes and possibly their variety, the central or heart wood is nearly jet black, as in the ebony. There is usually a marked difference in the color between the latest formed sap wood lying close under the bark and that formed many years before and now covered by later layers.

We have come now to consider another point of structure previously hinted at and plainly shown in the negravings, namely, the rings of wood. The tree as it enlarges from year to year leaves in its structure the evident record of its life. Each growing season is marked by a ring of wood, and only under the most adverse circumstances is this deposit omitted, and likewise extraordinary events only can lead to the formation of two rings. Therefore with a fair degree of certainty the age of a forest giant can be determined by the number of annual deposits of wood in rings around the common center.

These deposits become manifest to the naked eye, because of the difference in structure between the spring and autumn deposits, speaking of course for tree growth in the temperate regions. Glance at the papaw stem in Fig. 1, and it will be seen that the lower portion includes the heartwood nearly to the center of the stem. This is determined by the shortness of the diameter of the lowermost segments. It goes without further saying that the annual rings in exogenous (outside growers) stems vary in age from the youngest upon the outside to the oldest at the center. The point for us to determine is the lack of uniformity in the wood and why that lack is somewhat regular. In other words, the woody tissue of a stem is heterogeneous only within certain limits. Thus in the wood shown in Fig. 1 there are thirty-nine rings, and the tree for our purpose may be considered forty years old in round numbers. Twenty of these rings, or the older half, show a marked color, being much darker than the superimposed twenty years of annual deposits. Several other things are shown by this section, and we can well dwell upon this specimen, as it illustrates facts that are common to nearly all transverse

Fig. 5.—Cross Section of Pin Oak. Negative.

cuts of wood. The rings, for example, are not all of the same width, those formed while the tree was passing from its fifth to the twentieth season being the largest, but even among these there is a wide variation. Thus, ring fifteen from the center is a narrow one, followed by one of unusual width. For the last ten years the rings have been more uniform and much thinner than twenty years earlier. There may be one or more of many reasons for a ring being unusually thin, as, for example, a short season, one lacking in moisture or having an excess of it, injury from frost, fires, insects, or parasitic fungi. The decrease in thickness toward the outside of the papaw may be due to insufficient nutrition, approaching old age, etc., but in this connection it must not be overlooked that the amount of actual wood deposited may be more in a thin ring at the fortieth year than in a comparatively thick one at the tenth year, the surface covered being so much more extensive. It is likely that the root and leaf surface may not increase in the same ratio as that of the cambium or growing layer.

Let us now confine our attention to any one ring—the one, for example, near the middle of the engraving. It is bounded upon the inner and outer side by a dark line. Starting at the dark inner line, the ring of wood is very porous, as shown by the multitude of small holes giving a light appearance to this portion of the ring. Farther out the wood in the ring becomes more dense, until it ends in the almost solid outer dark band. This dense layer is in fact the last portion of the annual ring to be formed, and is laid down toward the end of the growing season. The next spring a new ring begins to form just outside this dense layer, and is often produced rapidly and with many large ducts and vessels among the woody fibers. In short, the ring of wood increases in density from the inside to the outside, and this being followed up year after year, the most dense or autumn wood is brought close to that which is the most porous, and the ring structure when seen in mass inevitably results.

It is not unusual for one side of a stem to grow faster than another, and then after a few years the center is toward one side of the middle, and the stem is called excentric. This is quite uniformly the case with all climbing stems, and the writer has a vivid recollection of a microscopic study of this subject of stem eccentricty in the poison ivy, for the work was interrupted by the swelling and closing of the eye most engaged in the task. Fig. 1 is still a fertile subject, and gives the observer a view of both this eccentricity and an irregularity not uncommon in stems. For some reason—and it might have been one of many—when the stem was about ten years old a defect developed, as shown upon the lower right-hand side, when each succeeding ring formed quite an angle that was gradually outgrown during the subsequent ten years. This blemish is shown perhaps to less advantage in the positive (Fig. 2).

The points that have been brought out in the papaw stem are also shown in the section of the ash. From what has been said it is evident that the lower side of the picture represents the inner side of the section. The center of the tree was where two pencils would intersect if held with their tips to the right and left side respectively of the lower edge of the engraving and at right angles to the curvature shown by the rings of growth. The tree from which the section used in the engraving was cut must needs have been at least a foot in diameter, but how much more can not be determined, for there is no means of knowing how far it is from the outermost ring shown to the bark. This could be determined in a general way from a knowledge of the ratio which obtains between the sap wood and the heartwood in this species.

The ash has certain peculiarities which separate it quickly from the papaw and most other woods. There is, in short, almost as much individuality in the woody tissues as in the foliage or flowers of many trees. Note, for example, the well-marked porous

Fig. 6.—Radial Section of Pin Oak.

portions, each ring being made up of two quite distinct parts, namely, the open vascular inner part and the dense fibrous outer portion. This arrangement of substance is conducive to that elasticity so characteristic of the ash, and, together with its medium weight, fits it for very wide and extensive service in implements and other ways.

There is another feature of woods, and one of great value from the artistic as well as economic standpoint, that the solar print illustrates. It is shown in some of its beauty in Figs. 1 and 2, while it fails quite completely in the ash namely, the thin, radiating bands which connect the center of the ste with the periphery and are known to botanists as the medullary rays, and to the workers in wood as the "silver grain." Fig. 4 is here introduced as showing this element of structure in a remarkable manner. The section is of the pin oak, and the lower right-hand corner represents for our purpose the center of the stem. The rings of wood are wide, irregularly scalloped, and show the points of structure previously mentioned in a superior manner. But best of all are the lines shot through the whole timber like rays of light (in the negative. Fig. 5) from the center to the circumference. They introduce another element, which up to this time has been left in the background. An exogenous stem may be said to consist of a central pith, seen best during the first years and often thereafter disappearing, and an outer ring of pithlike substance, the inner bark, and a series of plates connecting the two, also of the nature of pith. These thin plates separate incompletely the wood into wedges, and on account of them it often splits more easily in radial lines than in others, and may crack along them in ordinary drying. These thin, shiny, radiating plates of cells lying between the ordinary tissue of the wood give to some sorts of timber its beauty and value. Oak in all its strength would be lacking in much of its peculiar attractiveness were the silver grains absent. Fig. 6 shows a radial, longitudinal section of the pin oak with a few of these plates in view. They are usually small in area and appear in the finished article of furniture as shining, smooth patches, no two of the same size or shape. The beauty of this system of radiating plates is often enhanced by a curling and twisting, due to small knots scattered through the wood, as instanced in some sorts of maple, as the so-called "bird's-eye," a most attractive wood for finishing.

The birch is a good illustration of the wood being flecked, as shown in Fig. 7, a sample of the river birch. This wood is

Fig. 7.—Cross Section of River Birch.

peculiar in the absence of any conspicuous medullary rays, and of prominent vascular areas in the annual rings, and therefore with the exception of the pithy patches, the wood is quite uniform throughout; but the coloration characteristics of the heart may appear upon one side of the center like a radiating fan, thus showing that the change of color is far from constant, and does not depend upon the wood having reached a certain fixed age. Many other sections of wood might be shown, and each in its turn would exhibit peculiarities, but the purpose of the paper it is hoped has been attained—namely, to show engravings made from sun prints of thin sections of wood with the various elements of structure in the proper position and of natural size.

A single enlarged view of a section of the ash is herewith given, and both indicate the structure seen in Fig. 8 on a larger scale, and show that pictures of such objects may well be taken

Fig. 8. Cross Section of Ash. Magnified.

with the light passing through the object falling upon the sensitized plate in the dark chamber of the camera. By a comparison of Figs. 8 and 3 it will be seen that the two show the same ash wood in transverse section. In fact, a small portion of Fig. 3 near its center was selected for the picture from which engraving 8 was made, and this last is therefore no exception, for it was also a catching of a picture by Sunshine through the Wood.

In his subterranean explorations from 1888 to 1893, M. Martel has found that the temperature of natural caves is not equivalent to the mean annual temperature of the place, hut is inconstant; is not uniform in different parts of the same cave; and that the temperature of water in caverns is subject to the same variations as the temperature of the air, and is sometimes very different from the temperature of the air. The causes of these variations are not well understood, but as among them M. Martel mentions fissures admitting air from without; cavities in which cold air settles; and the influence of water, which cools the air through the evaporation of its oozings, or, when streams flow through the cave, brings in all the variations of the external air.