17^ PHILOSOPHICAL TRANSACTIONS. [aNNO 1787, 



of which precautions will render the event of the experiments precarious. I 

 have observed a very remarkable phenomenon, v/ith respect to the surface of 

 the mercury that is used for this purpose. It is, that though substances will 

 move on it with great nimbleness, when the mercury is first poured out of the 

 bottle into the open vessel, yet a short time after, viz. after having remained for 

 an hour or two, and some imes for a shorter time, exposed to the atmosphere, a 

 piece of brass or other substance will by no means move on it with equal faci- 

 lity ; so that some pieces of brass, or grains of platina, which, after first pour- 

 ing the quicksilver into the open vessel, were evidently attracted by the magnet, 

 about an hour after were not in the least attracted by it. The method which, when 

 the surface of the quicksilver is rendered thus sluggish, will effectually purify it, is 

 to pass the quicksilver through a funnel of paper, viz. a piece of clean writing paper 

 rolled up conically, and having at its apex an aperture of about a 50th part of an 

 inch in diameter ; which operation is sometimes necessary even on first pouring 

 out the quicksilver, and which I have often been obliged to repeat 3 or 4 times 

 in the course of an hour. There seems to be formed a kind of crust on the 

 surface of the mercury when exposed, which, though invisible by mere inspec- 

 tion, may be perceived by moving the floating body ; for if it be tried immedi- 

 ately after having passed the quicksilver through the paper funnel, the floating 

 substance will seem to proceed by itself; whereas, some time after, the same 

 body, when moved, seems to communicate that motion to the adjacent quick- 

 silver, and to drag it along with itself, somewhat like when one moves a body, 

 which floats on the surface of a liquor that begins to coagulate. 



The formation of this crust seems to be mostly owing to the imperfect metals, 

 which in various quantities are usually amalgamated with the common sort of 

 quicksilver ; because that amalgamation tends to dephlogisticate those metals, 

 and the semi-calcined part floats at the top, and it is not unlikely that the 

 dephlogistication goes on much quicker in the open air. The reality of this 

 supposition is corroborated by observing, that the purer the quicksilver is, the 

 smaller is the crust formed, or opposition made to the floating bodies. How- 

 ever, I have observed it in some measure even in the purest quicksilver that can 

 be procured ; and am inclined to think, that it must be partly owing to mois- 

 ture, or to very fine dust that adheres to the quicksilver when exposed to the 

 atmosphere. 



In performing such experiments, care should be had to keep the air and the 

 quicksilver as much undisturbed as possible, and also to present the magnet in a 

 proper manner, viz. so as not to touch the surface of the mercury, nor to strike 

 against the floating body, especially when that is in motion ; for after the im- 

 pulse, though that may be very slight, the floating body will be impelled back- 

 wards, which may be often mistaken for magnetic repulsion. The least excep- 



