For purposes of comparison it would be far better to have all contents of moisture determined at the boiling-point of water. While this varies with the altitude and barometric pressure yet it is quite certain that the loss on drying to constant weight at all altitudes is practically the same. Where the atmospheric pressure is diminished for any cause the water escapes all the more easily. This, practically, is a complete compensation for the diminished temperature at which water boils. Where the samples contain no ingredient capable of attacking aluminum, they can be conveniently dried, in circular dishes of this metal about seven centimeters in diameter and one centimeter deep, to constant weight, at the temperature of boiling water. =18. Moisture in Monocalcium Phosphates.=—In certain fertilizers, especially superphosphates, containing the monocalcium salt, the estimation of water is a matter of extreme difficulty on account of the presence of free acids and of progressive changes in the sample due to different degrees of heat. Stoklasa has studied these changes and reaches the following results[10]: A chemically pure monocalcium phosphate of the following composition, _viz._, CaO 22.36 per cent. P₂O₅ 56.67 “ “ H₂O 21.53 “ “ was subjected to progressive dryings. The loss of water after ten hours was 1.83 per cent; after twenty hours, 2.46 per cent; after thirty hours, 5.21 per cent; after forty hours, 6.32 per cent; after fifty hours, 6.43 per cent. This loss of water remained constant at 6.43 per cent. This loss represents one molecule of water as compared with the total molecular magnitude of the mass treated. A calcium phosphate, therefore, of the following composition, CaH₄(PO₄)₂·H₂O loses, after forty hours, drying at 100°, its water of crystallization. The calcium phosphate produced by this method forms opaque crystals which are not hygroscopic and which give, on analysis, the following numbers: CaO 24.02 per cent. P₂O₅ 16.74 “ “ H₂O 15.09 “ “ The temperature can be raised to 105° without marked change. If the temperature be raised to 200° the decomposition of the molecule is hastened according to the following formula: 4 CaH₄(PO₄)₂ = Ca₂P₂O₇ + Ca(PO₃)₂ + CaH₂P₂O₇ + 2 H₃PO₄ + 4 H₂O. The chemical changes during the drying of monocalcium phosphates can be represented as follows, temperature 200° for one hour: 8[CaH₄(PO₄)₂·H₂O] = 4CaH₄(PO₄)₂ + Ca(PO₃)₂ + Ca₂P₂O₇ + CaH₂P₂O₇ + 2H₃PO₄ + 12H₂O. The further drying at 200° produces the following decomposition: 4CaH₄(PO₄)₂ + Ca(PO₃)₂ + Ca₂P₂O₇ + CaH₂P₂O₇ + 2H₃PO₄ = 2Ca(PO₃)₂ + 4CaH₂P₂O₇ + Ca₂P₂O₇ + 2H₃PO₄ + 5H₂O. 2Ca(PO₃)₂ + 4CaH₂P₂O₇ + Ca₂PO₇ + 2H₃PO₄ = 6Ca(PO₃)₂ + 2CaH₂P₂O₇ + 5H₂O. Finally, pyrophosphate at 210° is completely decomposed into metaphosphate and water according to the following formula: