HQ FIIOST. [Lesson xx. 



its surface covered with oil of olives, does not 

 freeze so readily as without it : and nut-oil abso- 

 lutely preserves it under a strong frost, when olive 

 oil would not. The surface of water in freezing, 



C 7 



appears, as it were, wrinkled ; the wrinkles being 

 sometimes in parallel lines; but more commonly 

 they make angles of about 60 degrees with each 

 other ; and sometimes they are like rays proceed- 

 ing from a centre to the circumference. 



Another circumstance relative to the congela- 

 tion of water I cannot forbear mentioning, as it is 

 very remarkable. When water is cooled to within 

 eight or nine degrees of the freezing point, it not 

 only ceases to be farther condensed, but is actually 

 expandedby farther diminutions of its heat ; and 

 this expansion goes on as the heat is diminished, 

 so long as the water can be kept fluid : and when 

 it is converted into ice, it expands even still more, 

 and the ice floats on the surface of the uncongealed 

 part of the fluid. This is the more extraordinary, 

 as it is an exception to one of the more general 

 Jaws of nature with which we are acquainted. It 

 ie also worth while to remark, that though in tem- 

 peratures above blood-heat, the expansion of water 

 with heat is very considerable, yet in the neigh- 

 bourhood of the freezing point, it is exceedingly 

 small. Now it is evident, that when the specific 

 gravity of a liquid is but little changed by any given 

 change of temperature, the motions among the 

 particles of the liquid occasioned by this change 

 must be sluggish, and the communication of ht.at 

 of course very slow, and hence, from the preceding 



account 



