As the combustions of solids will no longer be a general method of obtaining heat, we shall greatly economise wood; and the wickedly mischievous word "inexhaustible" will not be applied to timber regions like the Rocky Mountain district of Canada. Arboriculture will become a more practical art than it as yet shows any signs of being; and along with careful afforestation will go skilled improvement in tree-growing. We shall replace all the trees we use by better trees, better cultivated. Even so, however, there will have to be devised great economies in the use of wood--economies like the recent invention of a method by which, instead of being wastefully sawn into planks, a tree-trunk can be cut up spirally, so that almost the whole of it may be used. In many places where wood is now employed in the arts, metals will doubtless be used instead, their greater neatness and durability making it advisable thus to substitute them, for reasons of convenience as well as economy; and probably new alloys, into which the lighter metals, as aluminium, will enter, may give us increased strength without increased weight, which will again be an economy, because it will save power. But even so, the world's expenditure of wood will continue to be enormous. War has been alluded to above. War is too wasteful, as well as too imbecilely uncivilised, to survive this century. It may be well to inquire as to the manner in which its abolition is most likely to be brought about. We may take it for granted that no sudden political or revolutionary movement will abolish the physical conflict of peoples. "All the arts which brutalise the practical polemist" will not be abandoned at a moment's notice on the bidding of any potentate or combination of potentates. To conceive of them as thus abandoned is to overlook the whole nature of political change. It is absurd (as Herbert Spencer remarks) to assume "that out of a community morally imperfect and intellectually imperfect, there may in some way be had legislative regulation that is not proportionately imperfect." But it would be equally absurd to believe that the moral and intellectual advance which our present tendencies show to be gradually taking place--an advance certain to be greatly accelerated during the middle half of the next hundred years--can fail to put a stop to war as a political device.