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This method of treatment is applicable to cotton fabrics as well as to papers. Woollens and silks require a slight modification of the process. These, when treated in the manner described with concentrated acids furnish a darkcolored, more or less fluid, mixture, which upon further heating fails to assume the dry granular appearance which is essential to the securing of a clear extract upon treatment with water. If this colored mixture is treated with water and filtered, a great deal of the organic matter passes through the paper, and the dark colored extract thus obtained is unsuitable for the subsequent test. In such cases satisfactory results have been secured by heating the material, as before described, with the concentrated acids till the nitric acid is expelled, and then adding, after the mixture has cooled somewhat, the proper quantity of paper which has previously been proved to be free from arsenic, and reheating. The mixture then readily assumes a dry granular appearance and upon treatment with water furnishes a clear extract.
The extract, obtained in the manner described, is tested for arsenic by the modification of the Marsh method known as the Berzelius-Marsh method. A failure to obtain a mirror within twenty-five to thirty minutes is regarded as proof of the absence of arsenic. All materials used in the process must be absolutely free from arsenic, and the mirror when obtained must be carefully tested in order to avoid any error which might result from the presence of antimony.
By means of the Berzelius-Marsh method, comparing the mirror obtained from an aliquot part of a solution obtained from a measured amount of paper or fabric with a set of standard mirrors obtained from known amounts of arsenious oxide, the amount of arsenic present can be easily and quickly estimated. This method for the quantitative estimation of arsenic was first suggested by Prof. H. B. Hill of Cambridge, and has since been worked out by Prof. C. R. Sanger, whose investigations are published in the Proceedings of the American Academy of Arts and Sciences, Vol. XXVI. Professor Sanger finds that it furnishes, when carried out with proper precautions, results worthy of comparison with other methods; and that it unquestionably surpasses all other methods for the quantitative estimation of minute quantities of arsenic.
The amount of paper or fabric taken for analysis has varied somewhat with the character of the material ; e. g., whether a plain color or figured. Usually 100 square centimeters (about 16 square inches), occasionally 200 square centimeters, have been taken. In the case of figured materials, it is important to secure all the colors in fair proportion. This can be effected by the use of patterns of different dimensions.
WILLIAM B. HILLS. Boston, Jan. 28, 1892.