Page:Proceedings of the Royal Society of London Vol 1.djvu/116

Rh 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 phaenomena 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 their effects to the radiant bodies? Several philosophers, such as Lambert, Saussure, and Pictet, have by various experiments deter- mined in favour of the aﬂirmzitive; 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 P 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 aﬂirmative 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 phas- nornena 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 molecules 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 equilibrium will be produced: if one of the spaces contain more ﬁre than the other, the changes will be unequal; but after a sufﬁcient time