G3 



night, or during ihe colder part of the year, at the same time 

 remairjing itself comparatively uniform in temperature. Even a small 

 lake frequently protects plants growing on its shores from injury, while 

 those at some distance may be killed by an early autumn frost. The 

 larger the body of water the more marked its equalizing influence will 

 be, and the greater the extent of country benefited by it. 



Generally bodies expand with heat and contract with cold. Water 

 IS no exception to this rule at the higher temperatures, but when below 

 4°C. it acts exactly opposite to this law. At this point, therefore, it 

 has its maximum density, i.e., is heavier than at any other temperature, 

 by this property water is still further preserved from variation, for the 

 heaviest portion (that nearest 4°C.) will remain at the bottom where it 

 is protected by the layers overlying it. 



A popular impression is that, owing to this pecul.arity of water 

 rivers and lakes are prevented from bei.ig frozen solid to the bottom 

 in winter. Although, in addition to the high specific heat of \va;er, it 

 doubtless helps to prevent this, the principal cause is to be sought lor 

 in the properties of ice. During its formation it expands very consi- 

 derably and, therefore, occupying more space than the water is lighter 

 than it and floats on the surface. Being a bad conductor of heat it 

 serves as a mantle, retarding very materially the action of the cold 

 atmosphere on the water. That ice occupies more space than water is 

 shown by the fact that when water is allowed to freeze in pipes or other 

 vessels they are very frequently broken by it. The heaving offence posts 

 and, to some extent, the bad roads in spring are also indications of this 

 property. Although doubtless sometimes doing considerable injury this 

 expansion of water when solidifying has been of immense value in the 

 formation of soils. 



Ice follows the general law of expanding with heat and contracting 

 with cold. The rolling, thundering noise sometimes heard on large 

 planes of ice, when the temperature is falling is caused by the contrac- 

 tion and subsequent cracking of ice. The fissures being filled with 

 new ice, the plane, on the advent of warmer weather, expands increasing 

 in area. The force with which this takes place is very considerable, as 

 frequently large stones are moved and heavy timbers broken by it. 



As the melting point of ice is always the same under ordinary 



