In 1923 the author indicated his belief that sound teeth contain more fluorine than carious teeth. His colleagues, Armstrong and Brekhus, subsequently confirmed this belief. When all calcium phosphate—necessary in a diet for good health in rats (even in absence of vitamin D)—is derived from fluor-apatite, there is no indication of fluorine poisoning; and general growth, and development of bones and teeth, are increased. Addition of 4 parts per million of sodium fluoride to the drinking water of mice does not mottle their enamel, and they survive the treatment. Since igneous rocks contain three times as many atoms of fluorine as of chlorine, a fluorine-free diet may be very difficult to prepare.
Reference: Rock Products, 26, 27, 1923.