In London the first cost has been about maintenance 18 cents per yard per year, the the end of seventeen years as good as new. total expense is 40 cents per yard per year.

$3.75 per yard, and the treet to be delivered at Including first cost, the

The success of this pavement in Europe gave rise to a demand for such pavements in America, but the expense of transportation of the rock from France was so great that inventors sought to find a substitute. They first tried the tar produced in large quantities at the gas works, which they erroneously supposed to possess the same qualities as the natural bitumen in the asphalt rock. This was combined with sand, limestone, sulphur, sawdust, etc. The material did not look unlike the real asphalt, and a craze for such pavements started in Washington in 1871, and spread all over the country.

The majority of these efforts were complete and costly failures, and, as they all claimed to be asphalt, the result was to create a prejudice against all pavements of that character, which it required years of careful experiment and proof to overcome. The defect of them all lay in the tar, which contained volatile matters which evaporated under the influence of the sun, and left the pavement a mass of dry black powder.

A Belgian chemist conceived the idea of using the asphalt of Trinidad as the cementing material, knowing that it had been exposed for centuries to a tropical sun, and that the sun's rays could have no further effect on it. With this he combined clean sharp sand and a small amount of powdered limestone. The sand in it afforded a firmer foothold for horses. It was used on a part of Pennsylvania Avenue, in Washington, in 1876; the asphalt-rock pavement of Paris was used on the other part, and they have been in constant use ever since. The French pavement proved more slippery and more costly than the Trinidad, and no more of it was laid; but the Trinidad asphalt gave entire satisfaction, and has been constantly laid with succeeding years, until now its area in Washington alone is but little short of one million yards. After seven years' successful use in Washington other cities began to use it, Buffalo being the first, and it now rivals Washington in extent of its use. It is now used in thirty-four cities, the total area being about four million yards.

These pavements are laid on a solid foundation of concrete six inches in thickness. The asphalt surface is two and one-half inches

thick, laid in a similar manner to those in Paris. But the asphalt is prepared quite differently. The refined asphalt is mixed with the residuum of petroleum to make the cement. This is heated at 300° Fahrenheit, and the sand is also heated to the same degree; these are mixed in a large box in which agitators are constantly revolving, and there a complete mechanical mixture is formed. The hot powder is then taken to the street, spread the proper thickness, and immediately compressed by large steam rollers, weighing from five to ten tons.

Of late years experiments have been made with other foundations than concrete. One of these is a mixture of broken stone and tar, forming a bituminous concrete. Another is the use of old well compacted macadam, thoroughly scraped and cleansed, and all depressions leveled up to a true surface. In both cases the object of the change is to effect a saving in cost. The bituminous base has been used in Washington, and the macadam in Chicago. Both appear to have been successful, but their complete success cannot be positively ascertained until they have been longer in use. The essential feature of the foundation is to have it solid and rigid. If any other material than concrete will secure this quality, of course it is equally good.

The cost of maintenance for the Washington asphalt pavements is about two cents per yard per year. On many of them, now ten years old, no repairs of any kind have been made, and they are still in perfect order. These streets are subjected to a very light traffic, and are kept clean. In other cities, where the pavements are not kept clean, and the traffic is heavier, in some cases the most destructive traffic in American cities, the expense is much larger. But under ordinary conditions the cost of maintenance does not exceed ten cents per yard per year for a long term of years. Including first cost the total expense of maintenance for seventeen years would be about 30 cents per yard per year. From this it appears that asphalt is a little more expensive than stone, but is the cheapest of the smooth pavements.

Asphalt blocks have been tried, but they are deficient in durability under heavy traffic, but they have been very satisfactory on residence streets of light traffic. These can be laid as ordinary paving stones, thus doing away with an expensive plant in every city.

On asphalt pavements the same force will draw a load three

times as heavy as on the ordinary stone pavement. The former can be kept perfectly clean at small expense; the latter has one-fifth of its surface composed of joints filled with stable filth, which cannot be removed in cleaning.

If some one gives voice to the current belief that horses are constantly falling on the asphalt, I will show him the result of careful observations in ten different cities on 736,000 horses, of which eightyfour fell on stone pavements, and only seventy-one on asphalt. The proprietors of the livery stables of Washington and Buffalo say that they invariably use the asphalt in preference to the stone pavement, and that there is far less injury to horses, as well as to their vehicles, on the asphalt.

At the close of the paper the President extended the thanks of the Society to Capt. Greene for his very instructive paper, and declared the meeting adjourned.

MEETING 386.

Artificial Fertilizers.

BY MR. WALTER 8. ALLEN.

The 386th meeting of the SOCIETY OF ARTS was held at the Institute on Thursday, March 14th, at 8 P. M., Prof. L. M. Norton in the chair.

After the reading of the records of the previous meeting the chairman introduced Mr. W. S. Allen, of Boston, who read a paper on "Artificial Fertilizers."

Mr. ALLEN said: The artificial fertilizer industry, now one of the most important of the chemical industries, is purely modern in its development, the foundation having been laid in 1840 by Liebig.

Liebig's theory, which stands today as the basis of all agricultural chemistry, may be expressed in a few sentences, as follows:

"The foods of all growing plants are inorganic substances. Plants live on carbonic acid, ammonia, nitric acid, water, phosphoric acid, sulphuric acid, silica, lime, magnesia, potash, and iron. Many need salt. Manures, in the form of animal excrements, do not work directly on plant life through their organic constituents, but indirectly by the products of their rotting and decomposition, in consequence, therefore, of the conversion of their carbon into carbonic acid, and their nitrogen into ammonia or nitric acid. Organic manures, which consist of parts or residues of plants and animals, may therefore be replaced by the inorganic compounds into which these residues separate in the ground.

The conditions necessary for a successful fertilizer are, concisely stated, these:

1. The fertilizer must contain one or more substances fit for plant food, and have its value only for such.

2. It must be concentrated, in order to lessen transportation charges.

3. It must be dry and finely pulverized, so that it may be evenly and easily spread.

4. It must contain its plant food in a soluble form, or in a form which will readily decompose and yield a soluble form.

5. It must not readily undergo changes by keeping, yielding a difficultly soluble or easily volatile form of plant food.

The basis of every form of artificial fertilizer is phosphate of lime, and to this may be added potash or nitrogen in some form. There are many sources of phosphate of lime, and the first of these, which found extensive use, was guano. This contained, also, ammonia and potash. The price finally became so great, on account of the gradual exhaustion of the deposits, that other sources of phosphates were sought. Bones, of course, were used, but the supply was limited. Attention was then turned to minerals containing phosphate of lime. Apatite and phosphorite are the two principal minerals which furnish the bulk of the phosphates. Apatite is found very widely spread over the world, but so far as this country is concerned the Canadian deposit near Ottawa is the most important. It varies in quality, containing as high as 90 per cent of tribasic phosphate of calcium, which constituent determines the price, varying, at Montreal, from $12 to $22 per ton.

The most important of the more crystalline phosphorites are those of the province of Estramadura, in Spain; those of the Duchy of Nassau, in Germany, known as the Lalui phosphorites; those of southwestern France, known as Bordeaux phosphates; those of northwestern France and Belgium, and the South Carolina phosphates of our own country. Among the other raw materials furnishing phosphoric acid the most important is the slag from the Thomas Gilchrist basic still process. This slag, which is rich in phosphate of calcium, is usually simply ground, and used in that condition as a fertilizer.

Many fertilizers contain potash, and at the present day the only source of this is the Slassfurth salts, which is found in central Germany.

When fertilizers contain nitrogen the sources of it are numerous. As nitrogenous materials found in commercial fertilizers the following may be named: nitrate of soda, sulphate of ammonia, dried blood, fish scraps, cotton-seed meal, linseed cake, castor-oil pomace, dried and pulverized scraps from slaughter houses, hoof and horn shavings, hair, leather scraps, shoddy waste, etc.

Great difference of opinion exists as to the relative advantages of these different forms of nitrogen, but as a rule the manufacturer uses that form which to him is cheapest.

As regards the relative value of sulphate of ammonia and nitrate of soda, field experiments indicate that each exerts a favorable influence on certain classes of plants. All nitrogenous fertilizers, before being taken up by the plant, are converted in the soil into nitrates. This is done by bacteria, and this nitrification accounts for the fact that nitrate of soda acts more quickly than sulphate of ammonia, although crops treated with the latter overtake the other in a few weeks.

As the value of a fertilizer depends entirely upon the amounts of phosphoric acid in the different states, potash and nitrogen, many methods of chemical analysis have been tried. The separation of the different forms of calcium phosphate which may be present in a fertilizer is difficult, and therefore chemists, working on the same material, but by different methods, obtained variable results.

To obviate this difficulty an association of the official State chemists was formed, which meets each year to revise these analytical methods, so that two chemists may obtain uniform results.