VOL. XXX.] PHILOSOPHICAL TRANSACTIONS. 439 



In all these experiments it is easily seen, that the effect is owing to the differ- 

 ence between the two attractions, by which mercury tends to glass and to its 

 own body ; these being always opposed to each other ; so that a particular ex- 

 plication is no way necessary. But perhaps it may save some little trouble to 

 the reader, to remove the following objection, which will readily occur to 

 him. 



In the experiments brought to demonstrate the 4th proposition, the globule 

 of mercury adheres to the glass in a plane surface, which cannot be done with 

 out increasing the surface of the globule, and consequently removing some of 

 its particles from the contact of each other. If therefore they tend more 

 strongly to each other than to the glass, why do they not recede from the glass, 

 and assume a figure perfectly spherical, that they may all have the greatest 

 possible contact with each other ? 



To this we may answer, that the power by which mercury is attracted, either 

 by glass or by other mercury, is proportional to the attracting surface ; and 

 therefore, though, caeteris paribus, the tendency of mercury to glass is not so 

 strong as its tendency to other mercury, yet in this case a much greater num- 

 ber of mercurial particles coming into contact witli the glass, than what recede 

 from the contact of each other, it is no wonder that the attraction of the glass 

 prevails, and causes the globule to adhere to it. For the number of mercurial 

 particles which lose their contact with the other mercury, is no more than what 

 makes up the difference of surface, which arises by changing the figure of the 

 drop : whereas the particles which by this means come to adhere to the glass, 

 are all those that constitute the plane surface in which the globule touches it. 



Which consideration ought likewise to be applied to the suspension of quick- 

 silver in glass tubes, either at extraordinary heights in the open air, or at lesser 

 heights in a vacuum, as above-mentioned. For the top of the tube being 

 spherical, or nearly so, it will be found that the contact of the mercury with 

 the extremity of the tube, is to the contact with other mercury, which would 

 be gained by its leaving the top of the tube and descending a very small space, 

 in a ratio infinitely great ; and consequently that the contact of the mercury 

 with the top of the tube is one cause of its suspension. 



Corol. 1 . — From this proposition it appears, that in a barometer made with a 

 narrow tube, the quicksilver will never stand at so great a height as in a wider. 

 Which accounts for the phenomenon so often mentioned by Mons. De la Hire, 

 in the History of the Royal Academy of Sciences at Paris, that in the baro- 

 meter which he constantly made use of for his annual observations, the quick- 

 silver did not rise so high, as in another he kept by him, by about 3 lines and a 

 half, which is near a third of an inch our measure : for he tells us that the tube 



