542 Jililhtit gfFhiUt Jiiring Trti"fmiJJi3ii of HtaU 



the motuvns of vwy fine particles of duft, accidentally mixed with the fpirlts of wine in Iii» 

 Jarge thermometer, and flrona;ly illuininntcd by the fun, that he firft dlfcovered the inter- 

 nal motions which take place in that (tuid during the time of its cooling. Rcftctting on 

 this facl, he immediately concluded that the internal motions of water might be rendered 

 equally viGble if he could fmd any foiid body of the fame fpeciiic gravity as water, and not 

 liable to be diflblved by it ; but fuch a fubtlance was not to be found. On re flcif\ion, it 

 occurred to him, that it is very fortunate that fuch fubflances do not abound; for othcrwifc 

 we fliould find great dilficuliy in procuring water in a pure (late. 



As the folid could not be found, he determined to adapt the fpecific gravity of the (lulj 

 to that of a folid very little heavier than the water in its pure (late. The folid he made 

 choice of was tranfparcnt yellow amber, of the fpecific gravity 1,078, and lie increafcd the 

 dcnfitv of the water by the addition of a certain quantity of pure alkaline fait. 



The amber was broken into pieces ^bout the fize of muftard-feeds, and put into a gljfs 

 vefTcl with water, to the bottom of which it funk. Upon the gradual addition of an alkaline 

 folution fome of the pieces rofe to the top, and others fubfided to the bottom. The former 

 were removed, and the fluid was carefully adjufted with regard to dcnfity, until the latter, 

 at 60° of Fahrenheit, remained permanently fufpended in the different parts of the fluid. 



A glafs vefll'l, confiding of a globe about two inches in diameter, with a cylindrical neck 

 three quarters of an inch in diameter and twelve inches long, was filled with diis prepared 

 liquid. The firft experiment made with this inftrument was to plunge it into a tall glafs 

 iar nearly filled with water almoft boiling hot. Two currents, in oppofite direilions, began 

 to move at the fame inftant with great celerity in the liquid in the cylindrical tube, the 

 afcendin" current occupying the fides of the tube, while that which moved downwards 

 occupied its axis. 



As the faline liquor grew warm, the velocity of ihefe currents gradually diminiflied \. and 

 at length when the liquor had acquired the temperature of the furrounding water in the 

 iar the motion ceafed entirely. On taking the glafs body out of the water, the internal mo- 

 tions of the liquor recommenced : but the currents had changed their direftions ; that 

 which occupied the axis of the tube being now the afcending current, ^\'llen the cylin- 

 drical tube was inclined a little, the afcending current occupied the upper fide, and the de- 

 fcendirg current the lower. Both ceafed when the inftrument had acquired the commoa 

 temperature of the room. 



In all cafes when the inftrument received heat, the current in the axis of the tube, or on its 

 lower fide if inclined, moved downwards; but when it parted with heat this motion was in 

 the oppofite direftion. A change of a few degrees was fufHcient to fet tlie contents of the 

 inftrument in motion. By the application of an heated body, the fluid neareft the place 

 of contaft was made to afcend ; but the contrary efie£l took pl.ice if the body were colder 

 than the inftrument. And in either cafe the motion thus produced was attended with a 

 contrary motion in fome other part of the inftrument. 



Among the applications made by this enlightened philofophcr to an inftrument fo calcu- 

 lated to deteft the fecrct operations of nature, one confifted in applying a lighted candle to 

 the middle of the tube while, it was inclined in an angle of about 45 degrees from the per- 

 pendicular. In this fituation the motion of the fluid in the upper part of the tube became 

 f.xceffively rapid, while that in tijc lower end, where it was united to the globe, as well as 



that 



