There is another important aspect of the dust problem to be considered. Until the general use of the automobile, most of the dust formed on the road was held on the surface until worked into the gutters by the action of rain and traffic, and was not a source of annoyance, except in extremely dry and windy weather. With the conting of the automobile, however, conditions have changed, and the dust problem has become more serious. While it is true that the wear caused by the passage of heavy rubber-tired automobiles is very slight, experience has proved that they are responsible for the removal and distribution over surrounding property of the dust formed by other kinds of traffic. (See Pl. XXII, fig. 1.) The facts that dust is a spreader of disease, and that along many heavily traveled roads it has caused so much annoyance and inconvenience as actually to depreciate the value of real estate, serve to emphasize the importance of meeting this problem with the serious consideration it deserves.

Although the facts stated above may be considered in a sense as a severe arraignment of the automobile in its relation to the public road, it should not be forgotten that there is another phase of the subject worthy of serious thought. The application of mechanical arts to the comforts and conveniences of civilization must inevitably bring up new problems, which can only be solved by patient experiment. While presenting new problems, these influences generally furnish the means of solving them. In this way the automobile, while tending to destroy macadam-road surfaces, has been an important influence, not only in the building of many miles of well-constructed highways, but also in rendering most urgent the study of road preservatives. The dust nuisance existed before the advent of the automobile, and if the experimental work now being done is successful it will be a demonstration of the really beneficial effect that the automobile has had upon the development of the art of road building. Many remedies have been suggested and tried for meeting this new condition, but a perfectly satisfactory solution of the problem is still to be found. Some success has attended the efforts of those who have sought to find a cure for the evil, and this is encouraging when the many difficulties to be overcome in the treatment of thousands of miles of roadway are considered. It is apparent that this problem can be solved only by the adoption of one or two general methods: (1) By constructing roads in such a manner and with such materials as to reduce to a minimum the formation of dust; and (2) by treating the surfaces of existing roads with materials that will give the same result.

Tar macadam and asphalt roads are good examples of the first method of construction, although the cost of these types of roads prohibits their general use for our country roads. Rock asphalt has been used in this country with considerable success, though to a very

limited extent, as a top dressing for macadam roads in place of the usual top coat of screenings, but here again the item of expense, except in localities near its source, generally prevents the use of asphalt on rural roads. In fact, the methods of construction mentioned, in so far as their general use at present is concerned, are chiefly confined to city streets. Without detracting from their value, where their use is practicable, it may safely be said that up to the present time no method of building an economical dust-proof country or suburban road has been devised. Whether it is possible for the problem to be solved eventually by following some method of construction in the first class mentioned remains to be seen, and it is by no means improbable that such a method may be found in spite of the difficulties to be met.

It is usual in treating the subject of dust preventives to classify them under a number of heads, but for some reasons it seems preferable to consider them according to the two general methods of their use on the road-those applied in their original condition, and those applied in emulsion or solution through the agency of water. It would be difficult to describe fully all of the preparations and methods of application which have been tried, and so only those of each type which have proved most successful will be considered in any detail. Many patents have been issued on special preparations, but satisfactory reports regarding their use are not available, and they will only be noticed to show the variety of substances which have been put forward as remedies for the dust evil.

Among the materials which are applied directly to the finished road surface without the agency of water, the mineral oils and coal tar are undoubtedly the most important. The value of these substances as dust preventives lies in the character and quantity of the "base" retained by the road surface after the more volatile constituents of the material have evaporated.

OILS AS DUST PREVENTIVES.

Among the mineral oils, those which contain the greatest amount of asphaltic base give the best and most lasting results, and chemical analysis will usually indicate those preferable in this respect. Some oils contain a paraffin instead of an asphaltic base, while others contain a mixture of the two. Owing to their greasy nature, paraffin oils are to be avoided as much as possible, and preference should be given to those containing asphalt, which acts as a good binder for the dust particles. The locality from which an oil is obtained is a general guide to the character of its base. Eastern oils, as a rule, contain an almost pure paraffin base; some of the Kentucky oils, and most of those found in Texas, have a mixed paraffin and asphalt base, while the California oils show an exceptionally high

amount of asphalt. In spite of the fact that California oils are far superior to the others, the cost of transportation prohibits their use in our Eastern States, where Kentucky and Texas oils must necessarily be used.

Oil for dust laying may either be applied in the crude state or be first subjected to fractional distillation at the refineries, where the lighter and more valuable products, used for illuminating and lubricating, are removed for use in the arts. An oil thus treated is called a reduced or residual oil, according to the point to which distillation has been carried. It is usually heavier and much more viscous than when in the crude state, and contains a greater proportion of the so-called "base." For this reason its use is in many cases considered preferable to that of the crude oil, although its high viscosity greatly increases the difficulty and consequently the cost of applying it to roads.

Oil has been used with some success, not only on macadam, but also on earth and gravel roads, and in this respect has its advantages over coal tar, which has so far given good results for the most part on macadam roads only. Many of the crude oils may be applied directly to the road surface by means of an ordinary sprinkling wagon, but when the oil is too heavy to be used in this way it may be applied with a suitable spraying device by the use of heat and pressure, or it may be run upon the road surface through a hose connected with the container. In some instances the oil has been applied directly to the road surface without previously removing the dust; in others, the road has been first swept clean of dust in order to allow the oil to penetrate as far as possible into the body of the road. When the latter method is followed the surplus oil which remains on the surface is usually covered with a light coat of gravel, sand, or rock screenings, and sometimes the dust which has been removed by sweeping is replaced. When a soft road is treated with oil the road is often harrowed to the depth of several inches and the oil is worked into the broken surface, which is then compacted by means of a roller. There are several patented oil spreaders which also work the oil into the surface of the road. A tamping roller has been used extensively in some of our Western States and has produced a very firm and even surface when used upon roads treated with California oil. This tamping roller not only compresses and packs the road, but also thoroughly mixes the oil with the earth. After the process of oiling and tamping has been completed, the surface is finished with an ordinary roller. (See Pl. XXIII, fig. 1.)

A number of experiments have been made in France and Algiers with petroleum, shale oil, and vegetable oils as dust preventives with satisfactory results, and similar results have, with a few exceptions, been obtained in this country. As a general criticism, however, it