TABLE 2 Data pertaining to the comparative efficiency of the pulp filter and the folded filter. (See also Table 1) the ammonium-phosphomolybdate precipitate, which is not absolutely insoluble in water. Inasmuch as the employment of the pulp filter enables one to accomplish both the isolation of the yellow precipitate, and its purification by washing with a small quantity of water, in but five minutes, it was reasonable to assume that the results would be the same whether the yellow precipitate is washed with ice-cold water or with water at room-temperature. Plimmer and Bayliss11 expressed the opinion that, with rapid filtration of the yellow precipitate by means of a special pattern of filtering tube, it is unnecessary to wash the yellow precipitate with ice-cold water. They did not give any data confirming their viewpoint, however. Below are reported the results of a number of phosphoric-acid estimations in which the yellow precipitates were washed either with ice-cold water or with water at room-temperature. The phosphate sol. contained 15 gm. of disodium hydrogen phosphate in 5 liters of water. All of the yellow precipitates, prepared in the manner mentioned above, were, after filtration, washed with 25 c.c. of distilled water; the washing was repeated twice. The results are summarized in Table 3. A glance at Table 3 shows that the yellow precipitates washed with ice-cold water required for their neutralization 5.03 c.c. of n/2 sodium hydroxid sol. (total average), while the yellow precipitates 14 Plimmer and Bayliss: Loc. cit. TABLE 3 Data pertaining to the solubility of ammonium phosphomolybdate in ice-cold water and in water at room-temperature For neutralization of the washed yellow precipitates n/2 NaOH sol, was used perature Water at room-temperature analy- Water at Ice-cold room-tem The Ave. washed with water at room temperature required 5.05 c.c. difference is so small as to be negligible. SUMMARY OF GENERAL CONCLUSIONS. its preparation and use, are described in detail. The paper-pulp filter, The efficiency of the pulp filter is shown. This filter not only combines the accuracy of the best paper-filters with the rapidity of the asbestos filter, but even surpasses both of them in some respects. The great economy of the pulp filter is demonstrated, its employment effecting a reduction in the use of filter paper to less than 25 percent. The advantageous application of the pulp filter to the filtration of the ammonium-phosphomolybdate precipitate is shown. STUDIES ON THE RELATION OF CEMENT DUST TO CITRUS VEGETATION 1. The effect on photosynthesis H. D. YOUNG* (Pathological Laboratory, University of California, Whittier) (Received for publication, June 20, 1915). INTRODUCTION. The effect of the dust arising from the rotary kilns, in the manufacture of Portland cement, on the surrounding vegetation has been the basis of considerable investigation. In Southern California it has become a problem of importance because of several characteristic factors. There are here some rather large cement-plants in the heart of a prosperous citrus district. As this is naturally a semi-arid climate and dependent on irrigation, there are no frequent rains to wash the dust from the leaves and, since the trees are not deciduous, the leaves with their dust-coating are not dropped annually. Because of the great value of the interests involved, the effect of the dust on the trees has recently been the subject of a great deal of litigation. In this connection the possible interference of the dust with photosynthesis, by excluding the light, was considered. Peirce1 found that orange leaves showed, in halves which had been freed from dust in the morning, "four or five times as many grains of starch as those in the covered and shaded half." As this would mean a very serious interference with photosynthesis, an investigation was undertaken to see whether or not it held true generally on dusty leaves. *The author wishes to express his indebtedness to the Riverside Portland Cement Company, Riverside, California, which defrayed the expenses of this investigation, and to Professor R. E. Smith, University of California, for his many helpful suggestions. 1 Peirce: "An effect of cement dust on orange leaves," Plant World, 1910, 13, p. 283. COMPOSITION OF DUST FROM ORANGE LEAVES. In order that there might be no doubt that the coating found on the leaves was actually composed of dust from cement-stacks, samples were collected and analyzed. In Table I are given the percentages of silica, ferric and aluminum oxides, and calcium oxide, for a sample of the "raw mixture" (the material fed into the kiln at the base of the stack), and of three samples of dust-coatings from leaves. These three samples were taken from trees approximately a half mile, a mile, and a mile and a half, respectively, from the plant of the Riverside Portland Cement Co. TABLE I Data pertaining to the composition of dust from stack and from orange leaves The figures in Table I agree closely enough to establish the fact that the dust on the leaves consisted chiefly of material from the kiln-stacks. AMOUNT OF DUST ON LEAVES. The amount of dust on the leaves varies greatly, of course. As has already been pointed out, there is little or no rain here from April to November, a condition that allows the dust to accumulate to a much greater extent than would be the case in a humid region. To determine the maximum amount deposited, a sample of 51 leaves was picked from trees quite close to the cement-plant in November, 1910. The total green-weight of the leaves was 29.96 gm. when picked; the total area was 1178.87 cm. The amount of dust was 4.04 gm. or 0.079 gm. per leaf. Av. wgt. of leaf 590 mgm. Av, area 23.11 sq. cm. Av. amount of dust Av. amount of dust per PERCENT OF LIGHT EXCLUDED FROM THE LEAF BY THE DUST. Since the primary object of this investigation was to determine the effect of the dust on photosynthesis, the amount of light excluded was first studied. Clear glass plates about 2" x 3" were placed in a tray and covered with water having cement-dust in suspension. After allowing the dust to settle for 24 hr., the plates were carefully removed and dried. Measurements were taken, with a LummerBrodhun photometer, of the amount of light excluded. The plates were then cleaned and the dust weighed. The data in this connection are summarized in Table 2. TABLE 2 Data pertaining to the amount of light cut off by a given amount of cement-dust on glass-plates From the figures previously given it will be seen that numbers 7 and 8 (in Table 2) have almost exactly the same weight of dust, per sq. cm., as the more heavily covered leaves. It is very probable, however, that the amount of light excluded by the dust-covered glass-plates is somwhat in excess of the amount excluded by the dust-covering of the leaves in the field, since the dust deposited from suspension in water is probably more coherent than that deposited in a natural way. The degree of exclusion of light by the natural dust-coating is very difficult to determine directly, since the opacity of the leaf itself and its coloration interfere. Considerable practice in such determinations, however, minimizes the importance of these sources of error. A series of determinations, using the leaves themselves, was made. A number of rather large, very heavily coated, leaves were taken, and the percent of light transmitted, before and after cleaning was determined, the difference being the amount of exclusion due to the dust. See Table 3. EFFECT OF SHADING ON CARBOHYDRATE SYNTHESIS. Carbohydrate synthesis in the leaf seemed to offer the easiest measure of |