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44 SALIVARY CALCULUS. thus charged is able to dissolve and hold these lime salts. * ' * * If by any process this carbonic acid is taken from the saliva, it cannot hold the salts and they are precipi- tated as tarter. * * The alkali that nearly, if not quite, always takes this carbonic acid from saliva, is ammonia. * # * The ammonia may be a result of the putrefaction of nitrogenous matter within the mouth." The statement of Dr. Watt that the ammonia may result from putrefactive change in nitrogenous matter within the mouth, means that the decomposition of meats and particles of other articles of food left between the teeth, results in libera- tion of enough ammonia to cause precipitation of the salts of lime from saliva, resulting in calcarious accumulations and crustation around and upon the teeth. There are some medical works that shed valuable light upon the subject of salivary tartar, rather indirectly certainly, but presenting facts of pathological nature that are applicable here. In Vol. I, of the " Reference Hand-Book of the Medical Sciences," 188o, may be tounu kt 1 . IIv, ,»"ln & language: " With regard to the immediate nature of the process involved in the precipitation of the lime salts, there is some difference of opinion. The simplest mode of explanation, and one which at present may be considered as nearly correct as any we have, is to look upon the process as similar to that involved in the formation of stalactites." All of us have noted the occasional incrustation that occurs on stones, and sometimes even on sticks and logs in streams and pools of water, the crustation varying according to the