404 On the CONTRACTION 



I am afraid that we cannot reft fatisfied with this explanation. 

 We muft not be deceived by the plaufibility of it. 



The ftate of perfect fluidity depends upon the circumftance, 

 that the particles of any body admit of ready motion upon each 

 other, and that the change of relative fituation meets with little 

 or no fenfible rcfiftance. 



Water certainly poffefTes fluidity in a great degree, and its 

 particles muft of courfe encounter but little refiftance, as they 

 glide the one upon the other. But if thefe particles (hall begin 

 to exert any degree of polarity, by which certain faces become 

 more difpofed to attach to each other than certain others, this 

 tendency would necefTarily oppofe that indifference with regard 

 to pofition, which is efTential to fluidity, and of courfe muft im- 

 pair the fluidity, and induce fome degree of tenacity or vifci- 

 ditv. 



To appearance, however, water at 32 ° has its fluidity as per- 

 fect as at temperatures confiderably elevated. Unwilling to truft 

 to appearance, where experiment might decide, I have attempted 

 in various ways to afcertain whether the water fuffers any fen- 

 fible diminution in this refpect while it is expanded by cold. 

 The following method I deem the moft correct. 



For the purpofe, I employed a gravimeter, the one contrived 

 by Mr Nicholson for difcovering the weight and fpecific gra- 

 ty of folids. 



This is a convenient inftrument, but, unfortunately, it is by 

 no means fo ticklifh as a balance. Duly loaded, fo as to be 

 equiponderant with the water in which it is plunged, Mr Ni- 

 cholson fays, it is fenfible to the 20th part of a grain. The 

 one I have, though its ftem be flender, is fcarcely fenfible to lefs 

 than two or three twentieths of a grain. 



The want of fenfibility in the gravimeter arifes, in a great 

 meafure, though not entirely, from a certain degree of tenacity 

 fubfifting among the particles of the fluid j and any thing that 



tends 



