152 ESSAY IX. 



heat be increased to full boiling, these particles divide 

 into white rare flocculi, and become almost imperceptible, 

 but without being dissolved, at least not entirely. I have 

 not been as yet able to collect a sufficient quantity of them 

 in order to determine exactly their nature. This, however, 

 I know, that, when exposed to fire, they are reduced to a 

 coal, which burns slowly to ashes, and is not soluble in 

 diluted nitrous acid. 



When calculus vesicle is dissolved in nitrous acid, no 

 precipitation ensues on adding the acid of sugar ; whence 

 one is readily induced to conclude that there is no 

 calcareous earth present, because this experiment is the 

 surest way to discover it. But I have found, in a variety 

 of experiments concerning elective attractions, that the 

 addition of a third substance, instead of disuniting two 

 already united, often unites with both very closely. That 

 the same thing happens here, I had the more reason to 

 believe, because the acid of sugar contains some phlogistic 

 matter, though of so subtile a nature, that, on being 

 burned, it does not produce any sensible coal ; and the 

 event of my experiment has shown that I was not 

 mistaken in my conjecture. In order to ascertain this 

 point, I burned coals of the calculus to ashes, which were 

 quite white, and showed in every respect the same 

 phenomena as lime, caused some effervescence during their 

 solution in acids, united with vitriolic acid into gypsum, 

 were precipitated by the acid of sugar, and were partly 

 soluble in pure water, etc. Notwithstanding this, there 

 remains about one-hundredth part of the ashes insoluble 

 in aquafortis ; the remainder of the above-mentioned 

 substance, which, together with the concrete acid, con- 

 stitutes the calculus. If the calculus be dissolved in 

 nifcroud acid, the solution filtered, and evaporated to 



