stitution, 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 cooling, 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 freezing point 
being 32 ° ; and this circumstance is of consider- 
able importance in the general economy of nature. 
The influence of the changes of seasons and of 
the position of the sun on the phenomena of ve- 
getation demonstrates the effects of heat on the 
functions 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 de- 
rive no nourishment from it. The activity of che- 
mical changes likewise is increased by a certain 
increase of temperature, and even the rapidity of 
the ascent of fluids by capillary attraction. 
This last fact is easily shown 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 discharged much more rapidly than 
the cold water.. The fermentation and decompo- 
sition of animal and vegetable substances require a 
certain degree of heat, which is consequently ne- 
cessary for the preparation of the food of plants j 
and as evaporation is more rapid in proportion as 
the temperature is higher, the superfluous parts of 
the sap are most readily carried off at the time its 
ascent is quickest. 
d 2 
