HEAT. 37 



being dissipated in the form of vapour or gas ; and the inter- 

 stices of the less volatile being thus emptied, the latter con- 

 tracts by its own cohesive attraction, giving thus & prima facie 

 appearance of contraction by heat. The pyrometer of Wedg- 

 wood is explicable on this principle. 



The second class of exceptions, though much more limited 

 in extent, is less easily explained. Water, fused bismuth, 

 and probably some other substances (though the fact as to 

 them is not clearly established), expand as they approach 

 very near to the freezing or solidifying point. The most pro- 

 bable explanation of these exceptions is, that at the point of 

 maximum density the molecules of these bodies assume a 

 polar or crystalline condition ; that by the particles being 

 thus arranged in linear directions like chevaux de fn'se, inter- 

 stitial spaces are left, containing matter of less density, so that 

 the specific density of the whole mass is diminished. 



Some experiments of Dr. Tyndall on the physical pro- 

 perties of ice seem to favour this view. When a sunbeam, 

 concentrated by a lens, is allowed to fall on a piece of 

 apparently homogeneous ice, the path of the rays is in- 

 stantly studded with numerous luminous spots like minute 

 air-bubbles, and the planes of freezing are made manifest 

 by these and by small fissures. Stars or flower-like figures 

 of six petals appear parallel to the planes of freezing, and 

 seemingly spreading out from a central bubble. These 

 flowers are formed of water. When the ice is melted in 

 warm water no air is given off from the bubbles, so they 

 seem to be vacuous ; it is, however, possible that extremely 

 minute particles of air sufficient to form foci for the melting 

 points of ice might be dissolved by the water as soon as it 

 surrounds them. Be this as it may, the existence of these 

 points throughout the ice, where it gives way to the heat 

 of the solar beam, if it does not prove actual vacuous or 

 aeriform spaces to exist in ice, proves that it is not homo- 

 geneous, that its structure is probably definitely crystalline, 

 and that the matter composing it is in different degrees of 

 aggregation, so that its mean specific gravity might well be less 

 than that of water. 



We cannot examine piecemeal the ultimate structure of 



