56 On the Terrestrial Arrangements connected with 



contracts in proportion as the cold increases ; but the instant 

 that it has cooled down as far as 39° Fah., it ceases to con- 

 tract, — ^nay, at a still lower temperature, it begins again to 

 expand, and continues to do so down to the freezing point. 

 The power of expansion is so considerable, that the strongest 

 metallic vessels, if completely filled with water, and closely 

 shut, are seen to burst during the process of freezing. This 

 power is indeed irresistible. I request you to keep this in 

 remembrance, since I shall afterwards have occasion to revert 

 to this subject. 



Water diminishes in volume, and gains in specific gravity, 

 in proportion as it continues to contract. Again, a heavier 

 fluid sinks below that which is lighter, as may be witnessed 

 on pouring water on oil. In the same way the heavier 

 particles of water descend through those which are lighter, 

 and the lighter ones rise through those which are heavier. 

 What takes place in a lake, for instance, in our Laacher 

 Lake, on the commencement of the winter's cold ? The 

 sheet of water on the surface being in immediate contact 

 with the cold atmosphere, begins to assume a lower tempera- 

 ture. It contracts, becomes heavier, and sinks down through 

 the water below, which, being warmer and lighter, rises in its 

 turn to the surface. This movement continues, until the water 

 which is uppermost has acquired the temperature of about 

 39° Fah. ; its specific gravity is then at its maximum. Water 

 of this temperature has, therefore, still a tendency to sink ; 

 but it loses that tendency the moment that it cools down 

 below 39° Fah. ; for now it begins again to expand, becomes 

 lighter, and swims on the warmer water below, as oil swims 

 on water. It follows from this, that water of a temperature 

 lower than 39° Fah., can never reach the bottom of the lake. 

 We have thus explained the mystery, why deep lakes can 

 never be frozen to the bottom. The temperature of water, 

 which occupies the lower regions of lakes, can never sink 

 below 39° Fah. ; whence we infer that, at a certain depth, 

 there exists a temperature of about 39° F., and this not only 

 in winter, but likewise in summer. I have said likewise in 

 summer, because it is obvious that water of the above tempe- 



