"Another important fact brought out was, that every variety of rock experimented on gave higher cementing results when com pressed while wet, which is analogous to the results obtained by road builders, who almost invariably find that stone screenings compact better when watered before being rolled. This at first led to the belief that this result was entirely due to a chemical change effected by the water; but briquettes made of pulverized glass, mixed with pure alcohol instead of water, gave practically the same results. The only explanation of this fact which at pres ent suggests itself is that any mobile liquid which will wet the fine particles of dust acts as a lubricant, allowing them to come in close contact when under pressure.
" By a process little understood, water has the power of attract ing the fine particles of rock dust and cementing them together. This is well illustrated when a drop of water falls on a dry hard road surface by the dust immediately buckling into an irregular shape, which is retained until destroyed by some force. On exam ining one of these little clods after drying, it will be seen that it sensibly coheres. The solidifying of mud by the drying up of puddles of water on clayey soil is another example, and so this same process can be traced even to the clay concretions. These phenomena may be due to totally different causes; nevertheless each is the cementation of rock dust, brought about by the pres ence of water, and in each case the finer the dust the more perfect this action. This cementation may be due to chemical action,
to physical re-arrangement of the particles, or more likely to a combination of such causes." Although chemical action does not seem to affect the ce menting power of stone dust as determined in the laboratory, it is probable that this force plays an important part in the road itself. Many native rocks consist of small bits bound together by a cementing material which was deposited from the water. Pure water will dissolve several of the common constituents of rocks, and its solvent action is materially increased by the acids which it takes up from the atmosphere and from manure and decaying vegetation on the road surface. Water percolating through the road material will dissolve lime, silica, and iron,— common cementing materials in natural rock,—and later deposits them in the interstices of the crushed stone, where they will act as a binding material. This binding action is quite slight, but may have an appreciable effect in maintaining the delicate adjustment of a broken-stone road. This subject has not been investigated, but it is apparently worthy of careful study.