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SALIVARY CALCULUS. 45 term of immersion and the geological character of the terri- tory in which the water was confined or through which it had passed. Water-pipes in our houses, and water-mains under cities become encumbered in time with incrustations of the lime salts that have been in solution in the water. Every one, probably, has recollections of a home in which as a child he enjoyed special privileges, among which was vis- its to the kitchen, where he was interested in watching the old tea-kettle whose lid danced apparently for his amusement as the steam sputtered out just as it did for the other Watt in the other century. One can recall the curious incrustations of lime that lined the tea-kettle's sides and bottom—much to the annoyance of the cook of early and blessed memory. The i lime that encrusts the stone in the brook, that lined the old tea-kettle, that obstructs the water pipes, is tartar ; not exactly salivary calculus, but tartar nevertheless, without the addi- tional elements of epithelium scales and general trash of in- ternal and extraneous origin. Continuing quotations from the " Hand-Book of the Medi- cal Sciences/' w e have the following : " A certain amount of T calcarious matter is a normal constituent of the blood in which it is held in solution by the carbonic acid always present in sufficient quantity to keep in solution twice the normal amount of earthy matter/' Herein we have full corroboration of the statement made by Dr. George Watt upon the same point. In the " Hand-Book of the Medical Sciences " from which quotations are made, we read further : " When the circulation is impeded, the free carbonic acid (because of its great diffus-