Having taken some pieces of this rock, which were quite free from the saline effervescence, and infused them in common spring-water for some hours, this water did thereby acquire the true taste of the Hartfell-Spaw. It likewise in the same manner received a deep blue tincture with galls, and contained all the other qualities of that mineral water, without the least difference, that I could observe: which, I think, ascertains the true origin of this mineral spring in the most obvious and undeniable manner. I am persuaded, that this plain and easy method of investigating the origin of mineral springs is not only superior to the most learned discussions and elaborate theories, but will be found to be the surest, yea, the only way of extending and compleating our knowlege concerning them. As I have not yet had the opportunity of making the experiments, which I designed, upon the two fossils, that we find to be the cause of the above mineral waters, and which will be necessary to elucidate and compleat these observations, which we have made concerning their origin; I shall now only add one thing, and recommend it to the observation of others: “_Whether or not, from such a knowlege of the origin of mineral waters, we may not acquire artificial ones of as great, or perhaps of greater, medicinal use, than those, which are naturally produced?_” XVIII. _An Account of the State of the Thermometer at the_ Hague _on the 9th of_ January 1757. _Extracted from a Letter of Mr._ Abraham Trembley, _F.R.S to_ Tho. Birch, _D.D. Secret. R.S._ [Read Mar. 3, 1757.] Hague, Febr. 15. 1757. I Carefully observed the thermometer during the cold days, which we have had this winter. I made use of the same thermometer, with which I made my observations in 1740, and for that purpose fixed it in the same place, where it was that year, _viz._ in a window directly exposed to the north, and open to a large square. In 1740 I saw Fahrenheit’s thermometer at two degrees below 0. This year, on the 9th day of January in the morning, it was at three degrees above 0; that is, only five degrees higher than in 1740. XIX. _Experimental Examination of_ Platina. _By_ William Lewis, _M.B. F.R.S._ [Read Mar. 17, 1757.] PAPER V. THE account of this extraordinary mineral, formerly read to this illustrious Society, and honoured with their approbation, being since published in the _Philosophical Transactions_, renders any recapitulation of the discoveries hitherto made unnecessary.