Apatite is a constituent of all rocks.2 Consequently its presence, to some extent, should be expected in every soil, and this expectation has been confirmed by the microscopic examination of a large number of samples of various types from many parts of the United States.s It is slightly soluble and is hydrolyzed by water, the natural apatites yielding an alkaline solution but one also containing phosphoric acid. It has therefore an importance in soil investigations. It is a carrier of an essential plant nutrient and it is often a guide to the genesis of a particular soil. In the investigation4 of the phosphates of lime which is in progress in this laboratory, apatite is being studied, and in this paper attention is directed to some observations which have been made upon this compound and the related substance, spodiosite. Apatite has been reported as made artificially in several ways: by the action of phosphorus trichloride on caustic lime,5 by passing hydrochloric acid over red hot tricalcium phosphate,' by melting tricalcium phosphate in sodium chloride,' and by melting tricalcium or trisodium phosphate in calcium fluoride or a mixture of these salts.8 It has also
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