The platinum standard has been adopted for use on the Boston Water Works. For converting any former readings on the Nessler or natural water scale to the platinum, the following table can be used. It was prepared from observations on fifteen. sets of natural water standards. TABLE FOR CONVERTING COLORS ON THE NESSLERIZED AMMONIA AND NATURAL WATER SCALES, TO EQUIVALENT VALUES ON THE PLATINUM SCALE. Natural Water Scale. Equivalent on Platinum Scale . 10 20 30 40 50 60 70 80 90 100 18 26 33 39 46 52 58 63 70 81 For standards darker than 100 no satisfactory comparisons have yet been made. For reading colors darker than 100 it has been found best either to read in shorter depths or else dilute with distilled water in order to bring the color within the range of the 100 standard. The latter method has been adopted on the Boston Water Works. The reason for this is that it is often difficult to compare the dark waters with the standard owing to a difference of hue. It has been found that the amount of light which passes through a number of equal layers of an absorbing solution, diminishes in geometrical progression as the number of layers increase in arithmetical progression. Thus if I denotes the intensity of the incident light, Ia will be the intensity after transmission through unit thickness, where a is a proper fraction, and depends upon the nature of the substance and the refrangibility of the light employed. For a given wave length, a will be different for different substances; and for a given substance, a will vary with the wave length. The quantity a is termed the coefficient of transmission.* It is because of the fact that the coefficient of transmis sion for the different rays varies with different solutions that we sometimes find a water matches the standard very closely in hue in a short depth, but appears of quite a different hue in a greater depth. This can be represented by a diagram as follows. Let the ordinates of the curve a b c * Thomas Preston, The Modern Theory of Light. in diagram, Plate 3 represent the intensities of the incident rays of light from the red to the violet, which fall upon a water, and the standard with which it is compared. Then assuming the following coefficients of transmission for a depth of 100 millimeters. The intensities of the different rays after traversing 100 millimeters would equal their original intensity multiplied by their respective coefficients. The intensities after traversing a depth of 300 millimeters would equal the original intensity multiplied by the third power of their respective coefficients giving the following: Red Yellow Green Blue Standard. Water. Violet Plotting these values, gives the curves in the diagram. The upper curve shows the intensity of the different colors in the incident light, and the four other curves show the intensities after having passed through 100 and 300 millimeters of the standard and water respectively. Comparing the intensities after passing 100 millimeters of the water with those of the standard for the same depth, it is noticed that they do not differ much in hue or luminosity. Comparing the intensities after passing 300 millimeters of standard and water, it is noticed that the hue of the water has approached the red, while that of the standard has approached the blue. These hues would therefore be quite different, and could not be accurately compared. The relative luminosity has also changed, the standard having become somewhat brighter than the water. Color readings made by different observers, have been found to agree to within 02. Occasionally differences of 05 or more are found. These differences are usually found in cases where the water from turbidity or other causes differs from the standard in hue. This renders the comparison difficult, and the result depends largely upon the judgment of the observer. The accuracy at present attained in color readings is probably quite sufficient for practical purposes. The principal difficulty in the way of greater accuracy in color readings is the difference in hue between the prepared standard and the water. Maxwell, Young and Helmholtz claim that all color perceptions are due to the simultaneous excitation of three sets of nerve ends in the eye, and that all colors can be produced by a combination of three colors, red, green and blue in the proper proportion. It is possible that a colorimeter might be constructed on this principle by employing standard red, green and blue solutions, so arranged that they could be combined in all proportions. A solution would also probably have to be employed to give the effect of turbidity to the standard. With a colorimeter of this kind much more time would be required to make the comparison. Prof. Ogden N. Rood* has employed these three constants to define completely a color. (1) Purity, or freedom from white light. . (2) Luminosity, or brightness. (3) Hue, or wave length. To measure the color produced by the absorption of a water it could be arranged so that the measurement of the first constant would not be necessary. The light after passing through the water, would consist of several components. It would be necessary to separate these by means of a prism or grating, and compare the spectrum thus obtained, with the spectrum of the original light. The measurements of the hues could then be accurately made, and the luminosity would remain as a photometric problem. * Text-Book of Color. By Ogden N. Rood. 1892. It appears, therefore, that in order to obtain much greater accuracy in color readings than is at present possible, more time and expensive apparatus must be employed. It would tend to uniformity of results and facilitate comparisons if all color readings were made on the uniform depth herein described, all water darker than 100 being diluted, as set forth on page 409. One of the purposes of the writers of this article is to urge all chemists, analysts and others interested in matters of water supply to adopt the methods herein described, unless some better method can be found. THE PHOSPHATES OF THE WORLD.* BY FRANCIS WYATT, PH.D. [Continued from p. 347.] The cost of producing one ton of rock in dry marketable condition, is at the present moment generally allowed by the river companies to be about $2.75 per ton, including fifty cents royalty to the State, and well-managed land companies with no royalty to pay, place their cost of production at from $2.75 to $3 delivered free alongside vessels in Charleston harbor. Of the total annual output, about two-thirds of nearly all the land-rock, are consumed in the United States, principally in Charleston, Richmond, Baltimore, Philadelphia and New York; the river-rock, which constitutes the remaining third, is exported. As a raw material of the first-class in the manufacture of soluble and available phosphates, South Carolina rock is everywhere held in the highest esteem. In Europe it is very popular, and, being of unvarying quality, has yielded results that cannot be surpassed by any other phosphate as an all-round staple, uniform and reliable article. *A Lecture delivered before the Franklin Institute, January 12, 1894. No absolute opinion can be expressed as to the probable extent and capacity of the yet untouched or unexploited land deposits and it would be manifestly absurd to attach undue importance to any estimate that might be formed. At the same time, my own investigations prompt me to suggest that it may probably extend over some thirty square miles. If the yield of this area were to approximate the present average of 750 tons to the acre, the conclusion would be that South Carolina may still produce about 14,000,000 tons of land phosphate. As the time rolls on the manufacturing requirements of the State will increase in large proportions, and I sincerely hope and believe that this source of consumption will eventually absorb all that can be produced, and leave nothing available for foreign markets. PHOSPHATE ROCK (WASHED PRODUCT) MINED BY THE LAND AND RIVER MINING COMPANIES OF SOUTH CAROLINA, SInce the |