406 Abstract of' Dr Hope's Address on 



next stated the various opinions entertained concerning tliem 

 and particularly respecting heat, and in historic order presented 

 the views of Bacon, Boyle, Boerhaave,Stahl, and Black, and ad- 

 verted to the discoveries of Black respecting latent and specific 

 lieat, and the successive labours of Irvine, Crawfurd, Wilke, 

 Magellan, Lavoisier and Laplace, Dulong and Petit, in the 

 same field. 



Heat presents itself in two very different conditions ; first when 

 combined with matter, pervading bodies slowly, either by com- 

 munication and conduction through and among its particles, or 

 by the movements of the particles themselves ; secondly, when 

 radiated, moving through elastic fluids or empty space with vast 

 velocity. 



The first of these had been studied by the philosophers al- 

 ready named, and not long after by Rumford. To the second 

 of these, viz. radiant heat, the subject of Professor Forbes's dis- 

 covery called upon him more especially to allude, and to pre- 

 sent a brief historic view. 



The radiation of cold, and its reflection by metallic^ mirrors^ 

 was known to Baptista Porta in the sixteenth century ; and ob- 

 servations were made on the radiation of heat, by the Florentine 

 academicians, towards the middle of the seventeenth century, 

 and by Marriotte in 168S. About the middle of the 18th cen- 

 tury, Lambert published his works on pyrometry and photome- 

 try, which contained some of the first accurate experiments on 

 this subject ; and the facts of the difficult transmission and re- 

 flection of heat by glass, was pointed out by the Swedish che- 

 mist Scheele. Pictet of Geneva extended his experiments on the 

 examination and the reflection of the heat derived from boiling 

 water, and our venerable associate Professor Prevost of the same 

 place, established the doctrine of the mobile equilibrium of heat, 

 in 1802. The triumph of this theory was found in the beauti- 

 ful experiments of Dr Wells, on dew, in 1813. 



Meanwhile, the experiments of Rumford and Leslie were 

 corroborating and extending these general views, even although 

 the doctrines of radiation were denied by the latter philosopher 

 in all his writings. The passage of radiant heat through solid 

 substances, such as glass, and through fluids, such as water, had 

 long been admitted, in the case where light accompanied heat. 



