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CHAPTER IV. TARTAR. Salivary calculus or tartar is universal in the mouths of man- kind, and probably in some degree common to all species that are provided with saliva. All authors upon this topic agree that salivary calculus is principally composed of certain salts of lime precipitated from saliva in the mouth, with various other accidental ingredients that become involved in resultant crust. It is rather difficult f to discover very much that is instructive written upon the eti- I ology of salivary calculus, but much of an ambiguous char- ( acter. It seems to the writer that the tartar so commonly observed in mouths of our patients, is so ordinary and com- monplace as to have attracted little important consideration. That this salivary deposit has not only an interesting etiol- ogy, but also important pnysiological properties there is abund- ant reason for believing ; also, the preceding chapters on pyor- rhoea alveolaris certainly justify conclusions as to pathological considerations. Dr. George Watt, of Xenia, Ohio, wrote in his little volume of Chemical Essays, as follows : " The lime salts named are probably present in all normal saliva—in solution. * * * Water is the chief constituent of saliva, and sub-phospate and carbonate of lime are but slightly, if at all, soluble in water. But normal saliva is saturated with free carbonic acid, and