^<^««t,^^;? ' sV their Classification J and their Uses. '> ^^- 523 



; 'If we examine these physical singularities with a view to their 

 classification, we shall find that some of them are the rigid con- 

 sequences of general laws with which we are acquainted. For 

 example, the existence of magnetic poles, that is, of points at 

 which the horizontal magnetic force vanishes, results from the 

 general character of the Potential surface. Again, the existence 

 of a position of minimum deviation is the necessary consequence 

 of the laws of prismatic refraction. 



There are, however, other physical singularities which are to 

 us, in the present state of our knowledge, absolutely anomalous. 

 We are unable to account, philosophically speaking, for their 

 existence; and as they appear to depend in general upon the 

 molecular constitution of bodies, there seems little probability 

 that the demands of science will ever be completely satisfied. 

 As a familiar instance, in cooling down from the point of ebul- 

 lition, water gradually contracts until it reaches the temperature 

 4F Cent., when it commences to expand, and continues to do so 

 until it solidifies in the shape of ice. The existence of a point of 

 maximum density is thus indicated for a liquid. A more remark- 

 able example of this class of physical singularities is furnished 

 by the expansion of Rosens fusible metal. This substance, an 

 alloy of bismuth, lead and tin, " contracts on cooling until it 

 solidifies at 75° ; after this it still continues to contract until it 

 attains to 55°, from which point to 35°, on the contrary, it ex- 

 pands, and after that contracts again, and continues to do so to 

 the lowest temperatures. It is to be remarked, that the volume 

 at the melting-point, 75°, is the same as at the point of relative 

 maximum, 35°. It is probable that this anomaly is connected 

 with some change which takes place in the arrangement of the 

 molecules of the metal consequent on the change from the liquid 

 to the solid state, and that it is analogous to the similar phseno- 

 menon in the case of water ; with this difierence, that in the 

 latter substance the anomaly begins to exhibit itself before the 

 change (solidification) takes place, but in the fusible metal not 

 until afterwards.'' (Dixon's Treatise on Heat, part 1, p. 41.) 



The expansion of this body may, in fact, be represented by 

 the curve 



where the abscissae represent the temperatures, and the ordinates 

 the corresponding volumes ; and we are now furnished with an 

 instance of a solid, exhibiting not only a point of maximum den- 

 sity, but also a point of maximum ^latation. I shaU discuss 



2M2 



