654 



ON THE COHESION OF FLUIDS. 



The square root of the rectangle .01, or .1, 

 19 the ordinate where the curve would become 

 vertical if it were continued ; but in order to 

 find the height at which the mercury adheres 

 TO u vertical surface of glass, we must dimi- 

 nish this ordinate in the proportion of the 

 sine of 25° to the sine of 45°, and it will be- 

 come .06, for the actual depression in this 

 case. The elevation of the mercury that ad- 

 heres to the lower horizontal surface of a 

 piece of glass, and the thickness at which a 

 quantity of mercury will stand when spread 

 out on glass, supposing the angle of contact 

 still 140°, are found, by taking the proporlion 

 of the sines of 20° and of 70° to the sine of 

 45°, and are therefore .0484 and .1330 re- 

 spectively. If, instead of glass, we employed 

 any surface capable of being wetted by mer- 

 cury, the heiglit of elevation would be .141, 

 and this is the limit of the thickness of a wide 

 surface of mercury, supported by a substance 

 wholly incapable of attracting it. Now the 

 hydrostatic pressure of a column of mercury 

 .0484 in thickness, on a disc of one inch dia- 

 meter, would be 1 3 1 grains ; to this the sur- 

 rounding elevation of the fluid will add about 

 ] 1 grains for each inch of the circumference, 

 with some deduction for the effect of the 

 contrary curvature of the horizontal section, 

 tending to diminish the height ; and the ap- 

 parent cohesion thus exhibited will be about 

 l60 grains, which is a little more than four 

 times as great as the apparent cohesion of 

 glass and water. With a disc 1 1 lines in dia- 

 meter, Mr. Dutour found it 194 French 

 grains, which is equivalent to 152 English 

 grains, instead of 160, for an inch ; a result 

 which is sufficient to confirm the principles 

 of the calculation. The depth of a quantity 

 of mercury standing on glass I have found, 



by actual observation, to agree precisely with 

 this calculation. Segner says that the depth 

 was .1358, both on glass anff on paper; the 

 dillerence is very trifling, but this measure is 

 somewhat too great for glass, and too small 

 for paper, since it appears from Dutoin's ex- 

 periments, that the attraction of paper to 

 mercury is extremely weak. 



If a disc of a substance capable of being 

 wetted by mercury, an inch in diameter, 

 were raised from its surface in a position per- 

 fectly horizontal, the apparentcohesioti should 

 be 381 grains, taking .141 as the height; and 

 for a French circular inch, 433 grains, or 628 

 French grains. Now, in the experiments of 

 Morveau, the cohesion of a circular inch 

 of gold to til e surface of mercury appeared 

 to be 446 grains, of silver 429, of tin 418, 

 of lead 397, of bismuth 372, of zinc 204, of 

 copper 142, of metallic antimony 120, of- 

 iron 115, of cobalt 8: and this order is the 

 same with that in which the metals are most 

 easily amalgamated with mercury. It is pro- 

 bable that such an amalgamation actually 

 took place in some of the experiments, and 

 affected their results; for the process of amal- 

 gamation may often be observed to begin al- 

 most at the instant of contact of silver with 

 mercury ; and the want of perfect horizon- 

 tality appears in a slight degree to have af- 

 fected them all. A deviation of one fiftieth 

 of an inch would be sufficient to have pro- 

 duced the difference between 440 grains and 

 528 : and it is not impossible that all the dif- 

 ferences, as fjir down as bismuth, may have 

 been accidental. But if we suppose the gold 

 only to have been perfectly wetted by the 

 mercury, and all the other numbers to be in 

 due proportions, we may find the appro- 

 priate angle for each substance, by deducting 



