FLUORIDE OF CALCIUM IN WATER. 153 



Moravia.* Middle ton has found it in the London pipe- water, and in three 

 ether waters, the localities of which he does not mention ; but as the experiments 

 were made in London, they were probably English. Traces of it have been found 

 in other waters also. 



I was induced to search for it in the water used in certain of the breweries 

 in Edinburgh, in consequence of learning that these rapidly corrode the ther- 

 mometers employed to regulate the temperature of the boilers and vats. The 

 fact was first mentioned to me by a gentleman, who, before I made any trials on 

 the subject, inferred that the corrosion of the glass must be owing to the pre- 

 sence of a fluoride in the water. I discredited the statement when I first heard 

 it, supposing that an incrustation or deposition of sulphate and carbonate of lime 

 had been mistaken for a true corrosion. I thought it impossible, moreover, that 

 fluoride of calcium, even if it were present, coiild act upon glass. But in the 

 course of the experiments already detailed, I had once occasion to notice that a 

 new Berlin porcelain basin, in which a considerable quantity of the aqueous 

 solution of fluoride of calcium was boiled down, had its glaze completely re- 

 moved. On observing this fact, I apphed to our intelligent instrument-maker, Mr 

 Stevenson, through whose hands the greater number of the thermometers used 

 by the Edinburgh brewers pass, in the course of receiving necessary repairs. He 

 informed me that he was quite familiar with the rapid dimming of the thermome- 

 ters, and that it was a true corrosion ; in proof of which he gave me two pieces of 

 broken thermometers, which I shew the Society. They are certainly abraded, 

 and present a surface like that of ground glass. The roughening which is so 

 manifest was not the result of friction against the sides of the brewing vessel, or 

 any other kind of mechanical action ; for the corroded part of the thermometer- 

 stem was enclosed in a brass-tube, and completely protected fi-om external 

 violence. It is proper to mention that the workmen in some of our breweries are 

 in the practice of scraping the stems of their thermometers, to remove the de- 

 posit of lime-salts which rapidly gathers on them, and are ready to aflirm that 

 the apparent corrosion is an abrasion occasioned by their own knives. To guard 

 against the possibility of any deception having occurred in this way, I visited the 

 brewery of Mr Campbell, situated in the Cowgate, behind Minto House, and was 

 shewn by his manager a thermometer which had never been scraped with any 

 instrument, and had been in use only a few weeks, but was nevertheless so 

 dimmed, that it required to be dipped into water in order to confer upon it a 

 temporary transparency, before the included mercury could be distinctly seen. 

 Mr Stevenson informs me that he finds the protected parts of the thermometer- 

 stems, which are enclosed in brass-tubes, as much corroded as those which are 

 exposed. 



* Bulletin des Sciences Medicales, vol. xvii. p. 425. From Zeitscliiift fiir Physik und Matheniatik. 

 VOL. XVI. PART IL 2 Q 



