172 On the Contraction of IVater ly Heat. 



stance, that ihe particles of any body admit of ready mo- 

 tion upon each other, and that the change of relative situa- 

 tion meets with little or no sensible resistance. 



Water certainly possesses fluidity in a great degree, and 

 its particles must of course encounter but little resistance, 

 as they glide the one upon the other. But if these particles 

 shall begin to exert any degree of polarity, by which certain 

 faces become more disposed to attach to each other than 

 certain others, this tendency vvould necessarily oppose that 

 indifference with regard to position, which is essential to 

 fiuidity, and of course must impair the fluidity, and induce 

 some degree of tenacity or viscidity. 



To appearance, ho-.vever, water at 32° has its fluidity as 

 peri'ect as at temperatures considerably elevated. Unwilhng 

 to trust to appearance, where experiment might decide, 1 

 have attempted in various ways to ascertain whether the 

 ■water sufiers any sensible diminution in this respect while 

 it is expanded bv cold. The following method I deem the 

 Bjost correct. 



For the purpose, I employed a gravimeter, the one con- 

 tcived bv iMr. Nicholson for discovering the weight and 

 specific gravity of solids. 



This is a convenient instrument, but, unfortunately, it 

 is by no means so ticklish as a balance. Duly loaded, so 

 as to be equiponderant with the water in which it is plunged, 

 Mr. Nicholson savs, it is sensible to the SOth part of a 

 grain. The one I have, though its stem be slender, is 

 scarcely sensible to less than two or three twentieths of a 

 grain. 



The want of sensibility in the gravimeter arises, in a great 

 measure, though not entirely, from a certain degree of 

 tenacity subsisting among the particles of the fluid ; and 

 any thing that tends to increase this tenacity, must, in the 

 same proportion, augment this want of sensibility. 



To ascertain whether any sensible chan2;e in the tenacity 

 or fluidity accompanies the expansion of water by cold, 

 ■which the theory requires, I examined the mobility of the 

 instrument when immersed in water at different tempera- 

 tures. I iirst plunged it into this fluid, heated to between 

 60" and 70". Under due loading, which sunk it to the 

 mark on the stem, it was not sensible to a weight less than 

 twn or three twentieths of a grain. 



I then tried it in ice-cold water, and found that its sen- 

 sibilifv was in no perceptible degree impaired. The cold" 

 ness of the water, it must be remembLred, causes some 

 ■degree of contraction of the gravini-;:lcr. 'lliis contraction 



cannot 



