gether upon the character and strength of the preservative and the care with which it is injected. It is probable that creosote does not penetrate the wood fibers, but merely forms an external coating around them; hence, in itself, it can not appreciably affect the strength of the timber. In general the ultimate strength of treated timber depends, first, upon the percentage of moisture remaining in the wood; and, second, upon whether or not the wood has been subjected to injuriously high temperatures during the preliminary processes of steaming and vacuum, if these processes were employed. The degree of temperature which can be applied without risk of serious injury depends upon the duration of the temperature, the moisture content and quality of the wood, and upon the pressure to which it is subjected."

If proper care is observed in the treatment of the timber all danger of injury from excessive temperatures can be avoided. The amount of moisture remaining in the wood is therefore a point of greater importance. As the moisture in a piece of wood is reduced by drying, the strength of the wood increases, and as moisture is subsequently reabsorbed, the strength up to a certain limit is again reduced. Creosote retards both the absorption and evaporation of water; hence its presence in thoroughly seasoned wood exposed to humid conditions tends to conserve its strength, whereas, on the other hand, if it is applied to green wood the strengthening action of waterevaporation is retarded. Some processes tend to increase the moisture content of the wood and others to diminish it. Therefore, whether the strength of timber is increased or decreased during creosoting depends chiefly upon the process employed.

The effect which live steam at safe temperatures has upon the moisture content of wood is now being made the subject of a special study. It is safe to say, however, that during steaming the amount of moisture in air-dry timber is increased, with a consequent decrease in strength, and that the succeeding vacuum fails to remove all of the added moisture before the introduction of the preservative. With many kinds of air-dry timber, however, the steaming can be dispensed with altogether, and this is done in many commercial plants. Whether or not it can be omitted with all kinds of wood is not yet certain.

These considerations, of course, do not apply to the open-tank method, or to similar processes where the timber is immersed

a Detailed discussions of these points will be found in Forest Service Circulars Nos. 39 and 108.

A discussion of the moisture content of creosoted wood may be found in Forest Service Circular 134, The Estimation of Moisture in Creosoted Wood.

directly in the hot preservative, without the preliminary steaming and vacuum. Immersion in hot oil tends to evaporate some of the moisture in the wood and so to increase its strength.

Zinc chlorid and the other preservatives which are in water solution have a wholly different effect. Unless the wood structure is already filled with moisture to the point of saturation, more water is injected into it with the preservative, with the result, if the wood is partially seasoned, of decreasing its strength. The original strength may be regained, however, by seasoning. If the zinc chlorid is injected into the timber in too concentrated a solution it may cause a chemical dissolution of portions of the wood fiber, with the result of permanently decreasing the strength of the timber. But for the solutions in common use this danger need not be taken into consideration.

Both creosote and zinc chlorid are excellent antiseptics, and both can be obtained in large quantities. Creosote's principal point of superiority, however, lies in its insolubility in water. Hence, once it is injected into timber it will not wash out, no matter how wet may be the situation in which the treated timber is placed. On the other hand, zinc chlorid is much cheaper than creosote, and since it is shipped in the form of a solid the freight charges are considerably less than they would be for enough creosote to treat the same amount of timber. But zinc chlorid is soluble in water, being in fact injected into the timber in water solution, and so when timber treated with zinc chlorid is exposed to moisture the leaching out of the salt is only a question of time. Hence, zinc chlorid is most commonly used in comparatively dry situations. Creosote, on the other hand, is used where the timber will be subjected to moisture. Moreover, creosote is one of the very few preservatives within commercial reach which offer absolute protection against the marine borers, which work such havoc among the wharves of the Atlantic, Gulf, and Pacific coasts. Since it is insoluble in water it can. not wash out of the piles into which it has been properly injected, and since it is more than a mere external coating there is no danger of its being broken off by floating débris.a

THE TENDENCY OF WOOD PRESERVATION IN THE UNITED STATES.

In the United States the tendency in wood preservation is to modify the processes rather than to change the preservatives. At present, creosote and zinc chlorid, pure or in mixture, are the only preservatives which are in genéral use. A constant effort is being made to overcome the chief drawbacks to the use of each of these.

a A more detailed discussion of this may be found in Forest Service Circular 128, Preservation of Piling Against Marine Wood Borers.

Processes have been developed which involve the compression and expansion of the air in the wood structure to expel some of the expensive creosote, leaving only a film along the cell walls. The amount of oil left in the timber is reduced, and in consequence the cost of the treatment as well. In other cases zinc chlorid and creosote are mixed together and injected into the timber in the form of an emulsion, the object being to reduce the cost of the treatment and to prevent the zinc chlorid from leaching out of the timber. Sometimes the timber

is impregnated with zinc chlorid, and only a narrow outer layer is filled with creosote; or again, glue and tannin are employed in the effort to plug up the outer wood cells and so keep the salt in the interior.

THE SAVING IN DOLLARS AND CENTS.

No process to preserve timber can come into use unless it is certain that the outlay for the treatment will be more than offset by the longer service of the treated timber. It is difficult to give a general example of the saving effected by treating certain timbers with preservatives, since it depends so largely upon local conditions and the class of timber. The following examples of the saving which under certain circumstances can be effected by proper preservative treatment are, however, typical, although they may not apply to other localities and to other kinds of timber without some modification:

An untreated loblolly pine fence post costs about 8 cents, or, including the cost of setting, 14 cents. Its length of life in this condition is about two years. Compounding interest at 5 per cent, the annual charge on such a post is 7.53 cents-that is, it costs 7.53 cents a year to keep such a post in service. If given a preservative treatment, which costs about 10 cents, the length of life of the post is increased to about eighteen years. The total cost of such a post, set, is then 24 cents, which, compounded at the above interest rate, gives an annual charge of 2.04 cents. Thus the saving due to treatment is 5.49 cents a year. Assuming that there are 200 posts per mile, there is a saving each year for every mile of fence of a sum equivalent to the interest on $219.60.

The saving due to treating railroad ties is also worthy of consideration. A loblolly pine tie untreated is worth about 30 cents, and its length of life in this condition is about five years. To this first cost should be added the cost of laying, which is about 20 cents. The annual charge figured as above is then 11.52 cents. If treated it will last for about twelve years. Its cost of treatment is about 35 cents. A treated tie in the track, therefore, costs about 85 cents. Compounded at 5 per cent, as in the above example, its annual charge is 9.48 cents. The saving per year is therefore 2.04 cents per tie. Assuming 2,880

ties per mile of track, the saving due to treatment alone amounts to $58.75 per mile, which corresponds to an investment of $1,175 per mile.

of 40 poles, to

Assuming that the cost of an untreated oldfield or loblolly pine pole, including hauling and setting, is $5, and that it lasts five years— a fair estimate for many portions of the United States-the annual charge, compounding interest at 5 per cent, amounts to $1.15. In other words, it costs the owner $1.15 a year for every such pole in his lines. This corresponds to a capital of $23 invested at 5 per cent interest, or, for a mile $920. Again, assuming that the butt of such a pole can be treated for $1, the first cost of the pole, set in the ground, is $6. The treatment may reasonably be expected to secure a service from the pole of twenty years, instead of five years when untreated. Thus, the annual charge on the treated pole, with the same rate of compound interest, is only $0.48 per pole, which corresponds to an investment of $9.60; or $384 per mile, as compared with the $920 per mile in the other case. Thus during the life of the treated pole a yearly saving of the interest on $536 will be effected for every mile of line.

It might be said that it is not positively proved that the treated poles will last twenty years, and that it will be necessary to wait until the poles are finally removed before the length of their service can be determined. A sufficient answer to this argument is that treated poles need to last only 1.6 years longer than untreated poles in order to justify the cost of treatment. Moreover, there is abundant evidence to show the long life of creosoted wood. Even in this country there are many examples of poles and other timbers creosoted twenty and even thirty years ago, which to-day are apparently as sound as when first set in the ground. In Europe, where wood preservation is an older industry, the results are still more marked. There have been failures, but in every instance they can be traced to incompetent or fraudulent work, insufficient impregnation, improper preparation of the timber, or some similar cause.

WHAT WOOD PRESERVATION CAN DO IN THE FUTURE.

At the present rate of consumption the exhaustion of the supply of structural timbers in the United States is a thing of the very near future. Moreover, the cost of fence posts is an ever-increasing burden upon the farmer and stockman. In the case of structural timbers telephone poles will serve as an example. Statistics gathered by the Forest Service show that in 1906 more than 3,500,000 telephone and telegraph poles were cut. This includes only poles 20 feet and over in length, and ignores the far greater number of poles and posts of

See Forest Service Circular 98.—Quantity and character of creosote in wellpreserved timbers.

smaller sizes. Of the poles cut, at least 40 per cent, or nearly 1,500,000, were either of white cedar or of arborvitæ.

a

Under average forest conditions it requires more than one hundred and ninety years to grow a 30-foot cedar pole. The average life of such a pole, when set in the ground in its natural state, does not exceed fifteen years. In other words, in order to meet even the present annual consumption there must be nearly thirteen trees growing in the forest for every 30-foot cedar pole standing to-day. A study of the rates of growth and the durability of other kinds of wood used for other purposes--ties, mine props, piling, etc.-shows that the consumption of structural timber greatly exceeds its production. Yet relief can be had in prolonging the length of service of the timber now being placed in position. Most of the cross-ties placed in the track to-day must be renewed within eight years. But if their life can be lengthened to fifteen years the benefits of the preservative treatments are plain. It is estimated that 150,000 acres are required each year to grow timber for the anthracite coal mines alone. The average life of an untreated mine prop is not more than three years. By proper preservative treatment it can be prolonged by many times this figure. Poles which in ten or twelve years, or even less, decay so badly at the ground line that they have to be removed can, by a simple treatment of their butts, be made to last twenty or twenty-five years. The same is true of fence posts and other timbers exposed to direct contact with the soil. Sap shingles, which are almost valueless in their natural state, can be easily treated and made to outlast even painted shingles of the most decay-resistant woods. Thousands of dollars are lost every year by the so-called "bluing" of freshly sawed sapwood lumber. This can often be prevented by proper treatment and at a cost so small as to put the method within the reach of the smallest operator. Millions of feet of insect and fire-killed timber in the West are standing untouched in the forest. Under present conditions this timber is not only useless, but is an actual detriment to the forest. Much of this dead wood possesses all the requirements of high-grade structural timber, with the single exception of durability. Often where dead timber is most abundant there is an almost complete absence of the naturally durable kinds of wood, and timber for structural purposes-ties, mine timbers, poles, posts, etc.-must be transported long distances at heavy expense. The thorough seasoning of several years has strengthened the dead wood and put it in an excellent condition for treatment. Fortunately, most of it is of a kind which readily absorbs the liquid preservatives, and so is well adapted to successful impregnation. Therefore, expensive

True for arborvitæ or white cedar in Michigan swamps. Western red cedar, a True for arborvitæ or white cedar in Michigan swamps. Western red cedar,