tube mill has been extensively used, but this arrangement has its serious disadvantages. In the first place the combination of a ball and tube mill requires an expenditure of about 12 H. P. per barrel of cement produced. In order to get an output of 14 barrels of cement per hour from a 5x22 feet tube mill it is necessary to feed the mill with material 90 per cent, of which will pass a 20 mesh sieve. In reducing material to this fineness in a ball mill it will be found that about 68 per cent. of the material delivered by the ball mill will pass 50 mesh sieve, about 50 per cent. will pass a 100 mesh sieve and about 45 per cent. will pass a 200 mesh sieve. After this material has been fed to the tube mill and samples taken of the product delivered by the tube mill it will be found that the fineness of the cement will just meet the specifications. To do this the mills have consumed an enormous amount of power, and owing to the crude method employed in reducing the material their cost of maintenance is relatively high.

In order to take advantage of the fine material delivered by the ball mill a number of air separating schemes have been tried and found wanting. These systems have failed simply because they rob the material of a certain amount of fineness, which although they may be light enough to be floated off in a current of air, have still not been reduced to the degree of fineness which will class them as impalpable powder.

A number of plants equipped with mills of the Griffin type, knowing that they could not meet the standard specifications for fineness, installed finishing mills in connection with their other types of mills. They now use their former finishing mills for preliminary mills and finish on some other type of mill.

QUALITY STANDARD ADOPTED.

This is the condition of affairs that confronted the committee appointed to standardize cement specifications. They all knew that the residue remaining on a 200 mesh seive is inert. They also knew that the pulverizing machines with which all the plants were equipped at the time were not capable of delivering a product having a fineness such that more than 75 per cent. would pass the 200 mesh sieve. They therefore fixed the amount required to pass the 200 mesh sieve at this figure. Since then, however, decided improvements have been made in pulverizing machines, and with the advent of the Fuller-Lehigh Pulverizer, the fine grinding of Portland cement has not only received renewed attention by the manufacturer, but engineers are beginning to specify cement having a fineness considerably in excess of that called for by the standard specifications.

It has been definitely proven to the entire satisfaction of both the manufacturer and the consumer that the fineness of Portland cement is a true measure of its cementing value. Only the extremely fine particles of cement passing the 200 mesh sieve are actively hydraulic. The greater the percentage of impalpable powder contained in the cement, the stronger will it be and the greater sand carrying capacity will it have. The finer the cement is ground the sooner will it attain a reliable strength. The hardening of cement is caused by the solution and subsequent crystallization of certain of its elements, and it is evident that the finer these elements are, the quicker will this hardening action be effected. The most important result of fine grinding however is the elimination of the liability to unsoundness possessed by all coarsely ground cements. The finer the particles are, the more readily will they become seasoned. All expansive elements will thus become inert, and complete crystallization will set in rendering the mass a unit. From present indications future improvements in cement manufacture will be directed almost entirely towards producing a cement whose physical qualities will be as certain as its chemical composition. The consumer will demand a product having a maximum percentage of active cementing material. He will object to paying for material containing 40 per cent. of inert matter, for it has been shown that even some of the cement passing the 200 mesh sieve has little hydraulic properties. The fineness of cement will no longer be gauged by its amount of 100 mesh or 200 mesh material, but by the percentage of impalpable powder it contains. Fine grinding is the solution of an absolutely uniform, physically reliable product, and when the standard specifications are revised, and revised they must be, they will recognize the merits of fineness and call for a product having a higher percentage of impalpable powder.

GROWTH OF THE INDUSTRY.

In order to obtain an idea of the development of the Portland cement industry, reference should be made to Table No. 4. This table shows the production of Portland cement in the United States from 1880 to 1907 inclusive, and also the production of Portland cement in Pennsylvania from 1890 to 1907 inclusive. These figures were obtained from the reports of the United States Geological Survey in various bulletins on the Mineral Resources of the United States. Supplementing this table is a diagram which graphically interprets the figures showing the development of the Industry during the years 1890 to 1907, inclusive.

It will be noted that during the period from 1890 to 1896 the production of Portland Cement in the United States increased about

AND IN

THE UNITED STATES FROM 1890 TO 1907.

460 per cent., and the production in Pennsylvania for the same period increased about 375 per cent. In 1897 the production of Portland cement, both in the United States and Pennsylvania, increased almost 100 per cent. This may in a measure be attributed to the introduction of pulverized coal for calcining purposes. The following year 1898 developed two interesting features. In this year the production of Portland cement in Pennsylvania exceeded the total amount of Portland cement imported into the United States from Europe. The total amount of foreign cement received in the various ports of the United States during the year 1898 amounted to 2,013,818 barrels, whereas the total production of Portland cement in Pennsylvania amounted to 2,095,141 barrels. The other point worthy of mention is that during this year the total production of rotary kiln cement exceeded the total production of dome kiln cement. This can be verified by referring to Table No. 3. From 1898 up to 1906 the growth of the industry has been marked by a steady increase in output, some years showing a production of almost 50 per cent. in excess of the previous year's output. In 1907 the increase in output both for the entire United States and Pennsylvania amounted to about 1,500,000 barrels. This small increase was due entirely to the abnormal economic conditions which prevailed during the latter half of the year, and should not be construed as an indication that the industry has attained its maximum development. The panic of 1893 had a material depressing effect on the industry in Pennsylvania, but as soon as normal conditions again prevailed, the demands on the Portland cement industry were such that it demonstrated beyond all question the fact that the market was broad enough to accommodate the product of every legitimate enterprise operating on a strictly commercial basis.

The curtailment of output during 1907 extended well into 1908 and has already materially affected the production of Portland cement for this year. It is the concensus concensus of opinion, however, among manufacturers, that as soon as conditions warrant the abolition of the general policy of retrenchment which, wide spread as it is, has affected the Portland cement industry less than it has any other branch of trade, it will find every cement plant having a ready market for its entire product, and the expenditure of millions of dollars will be required for the purpose of building new plants equipped with modern machinery in order to supply, in a measure the demand for material which will be created as soon as normal conditions prevail.

TABLE NO. 4-SHOWING OUTPUT OF PORTLAND CEMENT FROM THE UNITED STATES FROM 1880 TO 1907, AND FROM PENNSYLVANIA FROM 1890 TO 1907.

*Courtesy of George Otis Smith, Director U. S. Geological Survey.

THE CLAYS OF PENNSYLVANIA.

Clays of commercial value are found in every county of Pennsylvania. This could almost be said of every township. The deposits are of wide variety; some of them only a few inches thick, and the thickest upwards of a thousand feet. The area of one deposit may be less than one-fourth of an acre, and another hundreds of acres.