C 34 3 



There are a few exceptions to the law of expansion 

 of bodies by heat, which seem to depend either upon 

 some change in their chemical constitution, or on their 

 becoming crystallized. Clay contracts by heat, 

 which seems to be owing to its giving off water. Cast 

 iron and antimony, when melted, crystallize in cool- 

 ing and expand. Ice is much lighter than water. 

 Water expands a little even before it freezes, and it is 

 of the greatest density at about 41 or 42, the freez- 

 ing point being 32 ; and this circumstance is of con- 

 siderable importance in the general ceconomy of na- 

 ture. The influence of the changes of seasons and 

 of the position of the sun on the phenomena f vege- 

 tation, demonstrates the effects of heat on the func- 

 tions of plants. The matter absorbed from the soil 

 must be in a fluid state to pass into their roots, and 

 when the surface is frozen they can derive no nour- 

 ishment from it. The activity of chemical changes 

 likewise is increased by a certain increase of tempera- 

 ture, and even the rapidity of the ascent of fluids by 

 capillary attraction. 



This last fact is easily shewn by placing in each 

 of two wine glasses a similar hollow stalk of grass, so 

 bent as to discharge any fluid in the glasses slowly by 

 capillary attraction ; if hot water be in one glass, and 

 cold water in the other, the hot water will be dis- 

 charged much more rapidly than the cold water. The 

 fermentation and decomposition of animal and vegeta- 

 ble substances require a certain degree of heat, which 

 is consequently necessary for the preparation of the 

 food of plants ; and as evaporation is more rapid in 



