The sum total of these charges against wood as a pavement consists in the following: “Wood is porous, it is composed of bundles of fibres, it absorbs and retains wet, foul wet especially.” Why _foul_ wet should be absorbed more than ordinary wet does not transpire. There is no doubt that wood in its natural state does absorb a large quantity of water,[74] but this can be avoided in wood paving by preserving the wood of which it is composed by one of the following processes. (1) Burnetising Chloride of zinc is used in this process. (2) Kyanising Corrosive sublimate is used. (3) Renwickising Boiling in coal tar. (4) Boucherising Sulphate of copper is used. (5) Bethelising Creosote heated to 200° F. is used. (6) Seelyising Creosote is also used, the wood being first boiled. (7) Hayfordising Creosote is also used, wood being unseasoned. The fibres of the wood are also compressed, and no open joints between the blocks are permitted, by paving the blocks transversely, with butt joints closely packed together, and by filling the cross joints with an asphaltic or other impervious grouting. Wood paving should, however, be laid in streets with moderate traffic, and plenty of sun and air. In confined spaces such as courts, it soon rots and becomes a source of much unhealthiness.[75] Many reports have been from time to time made on the advantages and disadvantages of wood paving, and much has been said and written upon the subject, so that I will only touch upon some of the principal questions at issue. The first of importance is that of durability, and although the life of a hard wood constantly exposed to attrition is amazing, as may be seen on the stairs of the Metropolitan Railway Stations, and in many cog wheels of old machinery, still some diversity of opinion exists as to what may be fairly put down as the wear per annum of the surface of a street paved with wood blocks. It must be remembered that to arrive at any fixed ratio of wear, a standard of traffic should be fixed; but this unfortunately has not hitherto been done, so that the results of observations are bound to differ considerably. It must also not be lost sight of that the reason of excessive wear in a wood pavement generally arises from wide joints being the means of causing the edges of the blocks to abrade and become worn. Mr. D. T. Hope, in a paper he laid before the Scottish Society of Arts, upon some most careful investigations he had made on this subject, gives the wear as ¹⁄₈ of an inch in 18 months on blocks laid with vertical fibre, which he proved was the best manner of laying them to ensure the longest life. Mr. Deacon estimated the wear at from 1³⁄₈ inch to 2⁵⁄₁₆ inches per annum.[76] Mr. Copland estimated the wear at ³⁄₁₆ of an inch per annum.[77] Mr. Howorth estimates the life of wood paving at 25 years per inch of wood, if an absolutely uniform quality of wood fibre could be assumed.[78]