Burnett's process, which is favorably regarded in Germany, the preservative employed is the chloride of zinc (called Bur nett's fluid). The process as practised in Germany is described as follows: The cut cross-ties, to the number of thirty or forty, are piled on a light truck constructed for the purpose and run on rails into a large iron cham ber adapted to receive the same and to withstand the subsequent treat ment. The chamber is then sealed hermetically, and steam is admitted to it for about three hours, until the wood has attained the temperature of boiling water. The steam-siipply is now shut off, and the steam, together with the air displaced from the wood-cells by the injected steam, is per mitted to escape from the chamber by opening a valve controlling- the proper passage. The interior of the chamber is still further exhausted by means of an air-pinup until the pressure within has been reduced to one third of an atmosphere. By opening another valve, communication is established between the chamber and a vessel containing a solution of chloride of zinc. The latter is driven into the chamber by atmospheric pressure until it fills the chamber completely and forces itself into the pores of the wood. To make the penetration more complete, a force-pinup is applied until the pressure within the chamber reaches eight atmospheres. The impregnation is permitted to go on from one to six hours, at the end of which time the cross-ties may be withdrawn from the chamber.
Creosoting.—In the process of Bethell the impreg,nating materials employed are the oily products obtained from the distillation of coal-tar. As in the process just described, the penetration of the preservative agent is assisted by the employment of pressure. The process is commonly spoken of as " creosoting," from the fact that creosote is one of the princi pal constituents of the products -used. The process has been modified and improved by Seely, Hayford, and others in the United States, but in its general features the operation does not differ materially from that of " Bur nettizing," above described. The creosoting process is more expensive than the others, but the results obtained are now generally admitted to be better. In view of the expense, however, it may be preferable often to employ a cheaper process. In addition to the above named, various other fluid preservatives have been tried in this country, such as sulphate of cop per, or of zinc, and chloride of barium (Thilmany), chloride of zinc with glue, and tannin (We'Mouse), chloride of zinc and gypsum, etc.
The German railroads, on which the question of preserving cross-ties has been studied with the closest attention, have reached the conclusion that for impregnation creosote and chloride of zinc are about of equal value; but, as the impregnation with creosote costs about three times as much as with chloride of zinc, a majority of the companies emplov the lat ter. Of the sixty million ties laid on the German and ..A.ustro-Hungarian
railroads, it is estimated that nearly one-half are impregnated.
General Use of Preservative most European countries timber is scarce and correspondingly costly, aud, in consequence, its treat ment by one or another of the preservative methods above mentioned has been found very advantageous on the score of economy, and is almost uni versally adopted not only for railroad-ties, but also for engineering- struc tures generally.
In the United States the question of rendering timber more durable by preservative treatment formerly received little attention. The improvident destruction of the forest areas, however, is already beginning to make itself felt, especially in the older-settled sections, and of late the subject has received serious consideration. The conclusion reached by a committee of the American Society of Civil Engineers which reported ou the question in ESS5 was that " the time has probably arrived when in manv sections an economy of from twenty to fifty per cent. a year can be obtained in the maintenance of timber structures and cross-ties by artificially preparing them to resist decay, while in other districts timber is still too cheap to warrant spending money to preserve it." While approving of creosoting as the best proceSs to use, the committee's judg-ment was that its cost would preclude its use except in a few localities, and on this account, chiefly, the committee recommended Burnettizing as the most advisable process for railroad cross-ties.
When it is considered that there are in round numbers two hundred and fifty thousand miles of railroad in operation in Europe and America, having- on the averag-e two thousand cross-ties to the mile, of which it may safely be estimated that one-seventh must be renewed each year, it will be observed that the annual requirement of the railroads for cross-ties reaches the enormous total of 75,000,000 feet, not only involving a vast consump tion of timber, but also necessitating a larg-e annual expenditure of money. In the United States, the annual outlay for renewal of cross-ties is more than ten million dollars, and in Europe, where timber is much more expensive, this annual outlay of the railroads must be relatively much higher. In view of these facts, it is not surprising that attempts should be made to substitute materials other than wood for cross-ties.
string-ers (running lengthwise under the rails; it./.
20, ATS. 10, 12, 14, 15; pi. 24,fig. 9), which were quite common in the early period of railroading, are now rarely used save on street-railways in cities, for which form of service their disadvantages of warping, and of being difficult to bed substantially and to drain readily, are not so objectionable. To maintain the proper gauge of the track, however, it is customary to fasten the stringers to cross-ties laid between them.