present in the Waters of the Baltic. 347 
indeed, of the materials marked the glass distinctly, and four 
charges deeply. We may therefore infer that fluorine is 
present in the waters of the German Ocean, for different por- 
tions of the deposit yielded it readily, and marked glass as 
deeply as the deposit from the water of the Frith of Forth 
did, which could not have been the case if the whole crust 
had not contained fluorine pretty equally diffused through it. 
The results I have detailed, were communicated to the 
British Association at its meeting in 1849, and were con- 
firmed by Professor Forchammer, so far asthe Baltic is con- 
cerned.* At my request, he kindly furnished me with the 
letter which follows this communication, in which he an- 
nounces his discovery of the proportion of fluoride of calcium 
present in sea-water, as determined by analysis of that ob- 
tained from the Sound, near Copenhagen. The letter is other- 
wise interesting, and is printed in full. 
Since the results given above were communicated to the 
British Association, I have procured from Liverpool, a boiler- 
incrustation, from one of the Dublin and Liverpool Steam- 
Packet Company’s vessels, which, when treated with sulphuric 
acid, yields an acid vapour, readily corroding glass. From 
the same port, I have obtained a crust from the boiler 
of the “ Canada,” transatlantic steamer; and from Ports- 
mouth, a deposit from Her Majesty’s war-steamer, Sidon, 
which was three years on the Mediterranean station. Both 
of these crusts, after reduction to powder, yielded hydro- 
fluoric acid (or rather hydrofluosilicic acid) so abundant- 
ly, when treated with oil of vitriol, that a single charge 
of each, etched glass distinctly in two hours.t I can now, 
* Report of the Proceedings of Brit. Assoc., Atheneum for September 1849. 
t The crusts which give the best results, are those consisting of the most in- 
soluble salts of sea-water, which are found adhering to the walls of the boiler 
on which they have evidently deposited very slowly. They frequently contain 
a mere trace of chlorides, and but a small quantity of carbonates; their chief 
constituents being sulphates and fluorides of the alkaline earths, along with 
silica. Such crusts not only contain more fluorine than the looser deposits, but 
do not effervesce when heated with sulphuric acid, so that the hydrofluoric or 
hydrofluosilicic acid they evolve, is not swept away by carbonic or hydrochloric 
