660 



ON THE COHESION OF I'LUIDS. 



extreme inclination to the horizon is to tlie 

 inclination of" the upper surface, as the speci- 

 fic gravity of the oil to the difference between 

 its specific gravity and that of water : conse- 

 quently, since the contractile forces are held 

 in equilibrium by a force which is perfectly 

 horizontal, their magnitude must be in the 

 ratio that has been already assigned ; and it 

 may be assumed as consonant both to theory 

 and to observation, that the contractile force 

 of the common surface of two substances, is 

 proportional, other things being equal, to the 

 difference of their densities. Hence, in or- 

 der to explain the experiments of Boyle on 

 the effects of a combination of fiuids in ca- 

 pillary tubes, or any other experiments of a 

 similar nature, we have only to apply the law 

 of an equable tension, of which the magni- 

 tude is determined by the difference of the 

 attractive powers of the fluids. 



I shall reserve some further illustrations of 

 this subject for a work which 1 iiavc long 

 been preparing for the press, and which I 

 flatter myself will contain a clear and simple 

 explanation of the most important parts of 

 natural philosophy. I have only thought it 

 right, in the present Paper, to lay before the 

 Royal Society, in the shortest possible com- 

 pass, the particulars of an original investiga- 

 tion, tending to explain some facts, and esta- 

 blish some analogies, which have hitherto 

 been obscure and unintelligible. 



Vin. ADDITIONAL. EXTRACTS FROM LA- 

 PLACE, WITH REMARKS. 



In an essay read to the Institute of 

 France in December 1805, and published in 

 1806, as a supplement to the iMecanique ce- 

 leste, Mr. Laplace has advanced a theory of 

 capillary attraction, which lias led him to re- 



sults nearly similar to many of those which 

 are contained in this paper. The ceincidencc 

 is indeed in some respects so striking, that it 

 is natural, upon the first impression, to in- 

 quire whether Mr. Laplace may not be sup- 

 posed either to have seen this essay, or to liave 

 read an account of its contents in some pe- 

 riodical publication ; but upon further reflec- 

 tion, we cannot for a moment imagine a per- 

 son of so high and so deserved a reputation 

 as Mr. Laplace, to wish to appropriate to 

 himself an^' part of the labours of others. 

 The path which he has followed is also ex- 

 tremely different from that which I had 

 taken ; several of the subjects, which I had 

 considered as belonging to the discussion, 

 have not occurred to Mr. Laplace; and it is 

 much more flattering than surprising, that, 

 to an assembly of philosophers not extremely 

 anxious to attend to the pursuits of their co- 

 temporaries, investigations should be com- 

 municated, by themostdistinguished of their 

 members, as new and important, which had 

 been presented, a ^-ear before, to a similar 

 society in this country. In order to facilitate 

 the comparison of the methods which have 

 been adopted, I shall insert here a translation 

 of some parts of Mr. Laplace's essay, which 

 will also serve as an illustration of the theory 

 advanced in this paper; and I shall add some 

 remarks on the points in which those methods 

 differ most. 



" I have considered," says Mr. Laplace, " in the tenth 

 book of this work, the phenomena derived from the re- 

 fractive powers of transparent bodies acting on light. This 

 force is the result of the attraction of their particles ; but 

 tlie law of this attraction cannot be determined by the 

 phenomena, because they only require that it should be in- 

 sensible at all sensible distances. All possible laws of at- 

 traction, which fulfil this condition, agree equally well 

 with the different phenomena of refraction indicated by ex- 

 perience, the principal of which is the constant proper- 



