96 



mines the constant heat of a medium by the following proportion. 

 The difference between this heat, and each of the numbers given by 

 observation (that is, the initial and final observation), are to each 

 other as the first term of the progression is to the sixth ; that is to 

 say, as the numbers 13 and 10 raised to the fifth power. 



These comparisons between his results and those Dr. Herschel had 

 derived from the same experiments, have led our author to several 

 remarks, in which the above-mentioned law, and the circumstance of 

 the accumulation of heat in the intercepting media, are applied to 

 various phenomena and computations, and likewise to some experi- 

 ments of the same nature described by Prof. Pictet in his Essay on 

 Fire. The deviations here observed are in most cases ascribed to the 

 thickness of the intercepting substances, and to the distances between 

 them and the thermometers. 



The second part, which relates to the theory from which depends 

 the law of the increments of heat, as deduced from direct observations, 

 is introduced by a brief statement of the historical facts that have led 

 to the contemplation of this subject. Bacon first proposed the ques- 

 tion, whether heated bodies, which are obscure and opake, are similar 

 in then" effects to the radiant bodies ? Several philosophers, such as 

 Lambert, Saussure, and Pictet, have by various experiments deter- 

 mined in favour of the affirmative ; and it has even been proved that 

 the velocity of heat, independent of light, is no less than 69 feet in 

 an instant of time not apparently divisible. 



Bacon likewise asked whether cold might not, as well as heat, ac- 

 quire intensity by means of mirrors or refracting glasses ? Our author, 

 without mentioning the well-known experiments of the Academy del 

 Cimento on this subject, proceeds at once to those of Prof. Pictet, 

 who proved the affirmative as to the fact, but yet thought that the 

 cause ought to be ascribed not to the reflected cold, but to the re- 

 flection of heat in opposite circumstances ; by which he seems to un- 

 derstand that heat in this instance escapes reciprocally from the ther- 

 mometer towards the cooler substance. He here substitutes a move- 

 able equilibrium, to the immoveable one usually admitted by philo- 

 sophers ; and this he thinks fully explains the identity of the phse- 

 nomena according to his theory, which implies an equal apparent 

 dispersion of heat and cold. 



This theory is as follows : Fire is a discrete and agitated fluid ; 

 every molecule of free fire is moved with great velocity : some mole- 

 cules move one way, some another, so that a hot body throws out 

 calorific rays in every direction. And these molecules have sufficient 

 distance between them to admit two or more currents to cross each 

 other without being impeded in their course. This character of fire 

 being clearly understood, it must be evident (says our author) that if 

 we suppose two neighbouring spaces to contain a certain quantity of 

 it, there must be continual changes between them. If the fire is 

 equally abundant in each, the changes will be equal, and an equili- 

 brium will be produced : if one of the spaces contain more fire than 

 the other, the changes will be unequal ; but after a sufficient time 



