92 General Reflections on Heut. 
matter, without reflecting that this is also that principle by 
which tire itself, both animal and vegetable, is nourished and 
sustained. Dr. Black, at the close of -his incomparable lec- 
tures on heat, has noticed this subject, in bis asual plain but 
impressive language. * Its influence,” he observes, “is mani- 
festly so universal, and its action so liiportant and necessary 
to the progress of all the operations of nature, that, 10 those 
who consider it with some attention, it will appear to be the 
general material principle of all motion, activity. and life, in 
this globe. Heat is inseparably accessary to the existence 
of vegetables and animals. Without it they want the power 
to attract their nouristment, or to set it in motion through 
their system, or to refine and ripen it in their different parts. 
Their vigour and life depend on its influence. It is only when 
enlivened by heat that they make it assume the various 
forms and qualities, which we find in the wood, the root, 
the leaves, the juices, the fruit, the seeds, and the beautiful 
forms and colours displayed in the flowers. They decay and 
die when heat departs. Nor is animal life less dependent 
on heat for support, than vegetable. Heat is the main-spring 
in the corporeal part of an animal, without which all motion 
and life would instantly stop.” 
After referring us to the vivifying influence of heat exhi- 
bited in the incubation of an egg, the same excellent author 
proceeds: “ But, after the animal is thus brought into ex- 
istence, heat is still necessary for its support. If heat be di- 
minished toa certain degree, although no visible damage be 
produced, all motion and life are quickly extinguished. The 
animal is seized with a sleep and insensibility, under whieh 
it expires.” He goes om next to enumerate some of the 
endiess changes which occur in nature in consequence of va- 
riations of temperature, and thus concludes: * But, in this suc- 
cession of forms and operations which water undergoes, you 
will perceive that it is set in motion and adapted to these 
ends, by the nice adjustment, and gentle vicissitudes of heat 
and cold, which attend the returns of day and night, and sum- 
mer and winter. Were our heat to be diminished, and to 
continue diminished, to a degree not far below the ordinary 
temperature, the water would lose its fluidity, and assume 
the form of a solid hard body, totally unfit for the numerous 
purposes which it serves at present; and, if the diminution of 
heat were to go still further, the air itself would lose its elas~ 
ticity, and would be frozen to a solid useless matter like wa- 
