Animals pojfefs of producing Cold • 481 
Upon the revival of literature, the importance of this 
branch of fcience began very foon to engage the attention 
of philofophers. It could not efcape the general obfervation, 
in a penetrating and inquifitive age, when the powers 
of the human mind were employed with fo much ardour 
and fuccefs in exploring the operations of nature, that the 
element of fire ads a principal part in the fyftem of the 
world ; that by the influence of this element thofe motions 
are begun and fupported in the animal and vegetable king- 
doms, which are elTential to the production and prefervation 
of life ; and that it is the great agent in thofe fucceffive 
combinations and decompofitions, by which all things on the 
furface of the earth, and probably throughout the univerfe, 
are kept in a continual fluctuation. 
But though the utility of this branch of fcience was per- 
ceived, yet the progrefs that was made in the cultivation of it 
did not keep pace with the opinion which men entertained of 
its importance. Our fenfes inform us, that heat has a real 
exiftence, but they give us no direCt information with regard to 
its nature and properties : it is endowed with fuch infinite fub- 
tilty, that it has been called, by a very eminent philofopher, an 
occult quality : by fome it has even been confidered as an im- 
material being. It is, therefore, with great difficulty that it can 
be made the fubjeCl of philofophical inveftigation ; and hence 
the opinions of men concerning it have been fluctuating and, 
various, and the words which exprefs it vague and ambiguous. 
The firft ftep that was taken with a view to the cultiva- 
tion of this branch of fcience was the conftruCtion of a ma- 
chine for meafuring the variations of fenfible heat ; obferving, 
that heat has the power of expanding bodies, and considering . 
R r r the. 
