FLUORIDE OF CALCIUM IN WATER. 155 



rapidly corroded than those employed in the similar establishments in the Canon- 

 gate. As the action of the water may be supposed to be the same at both places, 

 and the attending circumstances similai', it must be some constituent of the Cow- 

 gate wells that occasions the difference. It may be the fluoride of calcium.* 



To conclude this part of the subject, I may state that Dr Christison informs 

 me, that he has frequently had occasion to notice that considerable quantities of 

 natural waters evaporated to dryness in glass basins, permanently destroyed the 

 transparency of the latter. From all that has been mentioned, it will ajjpear 

 that fluorine is likely to prove a frequent, though not an abundant, constituent of 

 ordinary water. If the proposal to construct the pipes of our water-works of 

 glass be put into practice, we may have an opportunity, on the large scale, of 

 testing the truth of this idea. 



It follows as a corollary, from the truths already detailed, that fluorine must 

 be present in sea-water. The inference that it must be there, had been drawn 

 by Mr Middleton from the fact, that fluoride of calcium occurs in the shells of 

 marine mollusca.f Silliman junior has come to the same conclusion, apparently 

 without a knowledge of Middleton's views, in consequence of invariably finding 

 the same fluoride in calcareous corals, t In the teeth of the walrus and of the 

 shark, the only marine animals I have examined, I found fluorine very distinctly, 

 especially in the latter. 



I attacked the problem, however, directly, by examining the water of the 

 Frith of Forth. A portion of the mother-liquor or hittern, from the pans at 

 Joppa, near Portobello (three miles from Edinburgh), in which sea-water is con- 

 centrated so as to yield culinary salt, was precipitated by nitrate of baryta. The 

 precipitate, after being washed and dried, was warmed with oil of vitrol in a lead 

 basin, covered by waxed glass, with designs on it. The latter were etched in two 

 hours, as deeply as they could have been by fluor-spar treated in the same way, 

 the lines being filled up with the white silica, separated from the glass. To the 

 acknowledged constituents of sea- water, fluorine, then, must now be added. 



* As the fact of the frequent presence of fluorine in water must hereafter enter as an important 

 element into all speculations as to the cause of the corroding action of water on glass, I place here on 

 record the result of an accurate quantitative trial on the latter subject. 



I am indebted to Mr John Adie for the particulars of the following experiment, which was made 

 with a view to discover what peculiarity in the structure of glass unfits much of it for optical purposes. A 

 cube of glass, two and a half inches square, was inclosed in a fir box, and fixed immovably in it by pieces 

 of wood. Holes were pierced in the sides of the wooden case, so as to permit the free passage of the 

 water, and the whole was placed in an engine-boiler, supplied with the Edinburgh pipe-water, and left 

 there for six months. During that period the boiler was in action twelve hours each day ; the water 

 being under a pressure of 35 pounds on the square inch, and at a temperature of about 260'^ Fahr. The 

 cube weighed, when first immersed, 9157 grains, and, when taken out, had lost 457 grs., or about ^Vth 

 part of its weight. It is right to mention, that the condensed steam was returned to the boiler, so that 

 fresh saline matter was only furnished in the water added, from time to time, to supply the waste. 



t London Phil. Mag., No. 144, p. 14. 



J On the Chemical Composition of Calcareous Corals, by B. Silliman junior. — American Journal 

 of Science, vol. i. Second Series. 



