38 CORRELATION OF PHYSICAL FORCES. 



matter, but in addition to the fact that the bodies which 

 evince this peculiarity are bodies which, when solidified, ex- 

 hibit a very marked crystalline character, there are experi- 

 ments which show that water between the point of maximum 

 density and its point of solidification polarises light circularly; 

 showing, if these experiments be correct, a structural alteration 

 in water, and one analogous to that possessed by certain 

 crystalline solids, and to that possessed by water itself, where 

 it is forcibly made to assume a polarised condition by the 

 influence of magnetism. 



The accuracy of these results has, however, been doubted, 

 and the experiments have not succeeded when repeated by 

 very experienced hands. Whether this be so or not, and 

 whether the above explanation of the exception to the other- 

 wise invariable effect of expansion by heat be or be not 

 regarded as admissible, must be left to the judgment and 

 experience of each individual who thinks upon the subject ; 

 at all events, no theory of heat yet proposed removes the 

 difficulty, and therefore it equally opposes every other view of 

 the phenomena of heat, as it does that which I have here 

 considered, and which regards heat as communicable expan- 

 sive force. 



As certain bodies expand in freezing,, and indeed in some 

 cases before they arrive at the temperature at which they solidify, 

 we get the apparent anomaly that the motion or mechanical 

 force generated by heat or change of temperature is reversed 

 in direction when we arrive at the point of change from the 

 solid to the liquid state. Thus, a piece of ice, at the tem- 

 perature of Zero, Fahrenheit, would expand by heat, and 

 produce a mechanical force by such expansion until it ar- 

 rives at 32 ; but then by an increment of heat it contracts ; 

 and if the first expansion had moved a piston upward s, the 

 subsequent contraction would bring it back to a certain 

 extent, or move it downwards, an apparent negation of the 

 force of heat. 



Again, with water above 39, i.e. above its point of maxi- 

 mum density, a progressive increment of cold or decrement 

 of heat would produce contraction to a certain point, and then 

 expansion or a mechanical force in an opposite direction. 



