148 PHILOSOPHICAL TRANSACTIONS. [ANNO 1781. 



Deity: they conceived it to be uncreated and immense, and attributed to its in- 

 fluence most of* the phenomena of* nature. Indeed, it is not wonderful that they 

 should have assigned it a high rank in the scale of being, as it was the great 

 agent which they employed in the chemical analysis of bodies, and was the in- 

 strument of those discoveries that attracted such universal admiration, and that 

 enabled them so successfully to impose on the ignorance and credulity of the 

 times. 



On the revival of literature, the importance of this branch of science began 

 very soon to engage the attention of philosophers. It could not escape the genera, 

 observation, in a penetrating and inquisitive age, when the powers of the human 

 mind were employed with so much ardour and success in exploring the opera- 

 tions of nature, that the element of fire acts a principal part in the system of 

 the world; that by the influence of this element those motions are begun and 

 supported in the animal and vegetable kingdoms, which are essential to the pro- 

 duction and preservation of life; and that it is the great agent in those successive 

 combinations and decompositions, by which all things on the surface of the 

 earth, and probably throughout the universe, are kept in a continual fluctuation. 



But though the utility of this branch of science was perceived, yet the pro- 

 gress that was made in the cultivation of it did not keep pace with the opinion 

 which men entertained of its importance. Our senses inform us, that heat has 

 a real existence, but they give us no direct information with regard to its nature 

 and properties: it is endowed with such infinite subtilty, that it has been called, 

 by a very eminent philosopher, an occult quality: by some it has even been con- 

 sidered as an immaterial being. It is therefore with great difficulty that it can 

 be made the subject of philosophical investigation; and hence the opinions of 

 men concerning it have been fluctuating and various, and the words which ex- 

 press it vague and ambiguous. The first step that was taken, with a view to the 

 cultivation of this branch of* science, was the construction of a machine for 

 measuring the variations of sensible heat; observing, that heat has the power of 

 expanding bodies, and considering the degree of expansion as proportional to the 

 increase of heat, philosophers have endeavoured by means of the former to 

 render the latter obvious to the senses. 



To this important invention, the author of which cannot be distinctly traced, 

 we are indebted for all the succeeding improvements in the philosophy of heat. 

 By means of it men were enabled to establish a variety of interesting facts, and 

 to bring some of the most obscure and intricate phenomena of" nature to the 

 test of experiment. The opinion, that the heats inherent in various heteroge- 

 neous substances differed from each other in kind, as well as in degree, was now 

 exploded, since all were found to produce similar effects on the thermometer. 

 The increase and diminution of temperature in the different seasons and cli- 



