346 On the Proportion of Fluoride of Calcium 
fluoric acid ; and the latter is thus swept away, before it has 
time to corrode the glass deeply. I preferred, nevertheless, 
to use the crust exactly as I got it, that the proof of the pre- 
sence of fluorine might not be impaired in validity, by the pos- 
sibility of that substance being introduced in the water and 
re-agents, which must have been employed had the chlorides 
and carbonates been separated from the crust by a preliminary 
process. The crust, accordingly, after having been dried and 
powdered, was placed, along with oil of vitriol, in a lead basin 
covered by a waxed square of plate-glass, with letters traced 
through the wax. A single charge of the crust corroded the 
glass only slightly, but by replenishing the basin with suc- 
cessive quantities of the powdered crust and acid, whilst the 
same plate of engraved glass was used as the cover, I found 
no difficulty in etching the glass deeply. Iam indebted to 
my friend Mr Stevenson Macadam, for this simple but effec- 
tive way of increasing the corrosion of the glass, which seems 
worth the adoption of chemists in all cases where fluorine is 
sought for. Four charges of material have been sufficient, 
in all the specimens of sea-water deposit I have examined, 
to mark the glass strongly. It was kept wet on the upper 
side, and exposed undisturbed to the action of each charge 
during twelve hours. 
Operating in this way, I have found fluorine readily in the 
boiler-deposit from the waters of the Friths of Forth and 
Clyde. It is a less easy matter to subject the waters of the 
open sea to the requisite concentration before examination. 
It occurred to me, however, that the incrustations which are 
periodically removed from the boilers of the ocean steamers 
would serve to determine the question, whether fluorine is a 
general constituent of the sea. 
I have obtained accordingly at Leith, the crust from the 
boiler of the S¢ Kiaran, which traded between that port and 
Wick, so that the greater part of the water consumed as 
steam by its engines is derived from the German Ocean, al- 
though a portion is necessarily obtained from the Frith of 
Forth. The crust from the boilers of this vessel, was treat- 
ed in the way described, and at once yielded hydrofluoric 
(and probably also hydrofluosilicic) acid. A single charge, 
