THE PRESERVATION OF WOOD FROM DECAY. 81 
efficacy of each process ; secondly, th'e kind of wood to be used, and its condi- 
tion ; thirdly, the expense, both actual and comparative ; and fourthly, the par- 
ticular use to which the wood is to be applied. The first and second of these 
questions can be settled only by the evidence of actual tests. The matter of 
expense involves, to some extent, the subject of locality. The fourth question 
opens up considerations of this sort. Given two processes for preserving wood,, 
one of which will protect for fifteen years and the other for only ten, the latter 
being decidedly cheaper. If, now, we are to deal with the timbers of a bridge, 
the first of these processes, notwithstanding its greater expense, may be the more 
desirable of the two. But if we are to lay down wooden pavements which will 
be worn out by the wear and tear of travel long before the cheaper preservative 
has lost its protecting power, then the latter, though the inferior process in gen- 
eral, is the better for our purposes. In other words, an expense which may be 
advantageously incurred in one case, may be wholly unadvisable in a second. 
The decay of wood may be generally traced to one of three causes. It is 
due either to slow oxidation, to the ravages of certain minute animals, or to an 
action induced by contact of the fibre with the decomposing albuminoid sub- 
stances of the sap. It has at times been ascribed to the growth of fungi; but as 
these have been found to appear only after decay has fairly commenced, this sup- 
position may be set aside. The rapidity of the change, however, is much influ- 
enced by external circumstances. In perfectly dry places wood rots very slowly^ 
and has been known to remain sound for hundreds of years. Completely immersed 
in water, except where it is exposed to the attacks of the teredo, it is similarly per- 
manent. The piles of Old London Bridge were found to be good after having 
been down eight hundred years. But in damp places, or in places alternately 
wet and dry, especially where there are frequent and great changes of tempera- 
ture, wood decays very quickly. This is the case with wooden pavements par- 
ticularly. Snow, rain, sun-heat, and frost are ever at work upon them, and par- 
ticles of fermenting animal matter, like horse-manure, are constantly getting in 
between the blocks, and making the difficulty of preservation greater. 
Now, leaving out of account altogether the processes of drying, washing, 
boiling, and steaming wood, we shall find that four distinct classes of methods for 
its preservation have been proposed. The first class contains all those methods 
which deal simply with the surface of the wood, leaving the interior structure 
unprovided for. Every process of this sort consists merely in the application of 
some air-tight varnish to the wood, none of these varnishes being sufficiently 
valuable to warrant description here. ^ The second class of processes comprises 
those by which the surface of the wood is carbonized, and thelayers immediately 
beneath are somewhat affected also. Three plans of this sort have been proposed. 
In the first, which has no recommendations, the wood is charred by immersion 
in strong sulphuric acid. The second plan is to dip the wood into mineral oil or 
naphtha, then, after withdrawing it, to kindle its surface, and allow it momentarily 
1. The recipe for such a varnish may be found in Scientific American, Vol. VI. 
