Mr. G. F. Rodwell on the Theory of Phlogiston. 15 



being unhampered by the presence of a large quantity of the 

 third element is present in sufficient quantity to give motion to 

 the globules of the second element associated in large quantity 

 with the substance, but is unable to move the particles of the 

 third element. Heat is apparent without light (as in the case of 

 all sensibly hot non-luminous bodies, and to a less extent of all 

 matter), when the materia coelestis has sufficient power to move 

 the particles of the third element, but is so entangled in them 

 that it cannot move the globules of the second element. Heat 

 and light are simultaneously produced (as in the case of flame, 

 and in all instances of quick combustion) when, there being 

 many globules of the second element present, the materia coelestis 

 exists in such quantity that it is able to communicate motion 

 both to the second and third elements. The particles of_oils, 

 sulphurs, and combustible bodies generally are peculiarly disposed 

 to receive the motion of the materia coelestis ; and such bodies 

 are said to be "potentially hot." A nail which is being driven 

 into a block of wood does not grow hot until after it has been 

 driven home by the hammer, because heat is the motion of the 

 insensible parts of matter, not the motion of masses ; and as 

 long as the nail itself is capable of moving, the force of the blow 

 is expended in producing that motion of a mass, not in moving 

 the insensible parts of the body. Light consists of globules of 

 the second element agitated by the materia coelestis ; there is no 

 transmission of substance, simply of motion. Lux is to be dis- 

 tinguished from lumen : the former is a propensity to motion, 

 an action which tends to induce motion ; the latter is the motion 

 communicated to a medium. Flame tends upwards because it 

 contains a large amount of materia coelestis, which is lighter than 

 air, and which is the cause of lightness in all bodies*. 



The physical philosophy of Descartes slightly resembles that 

 both of Epicurus and of Aristotle ; it will be observed that the 

 " materia coelestis" in many respects resembles the " afflrjp" of 

 Aristotle ; and it is impossible to read the fourth book of the 

 treatise ( ITept Ovpavov ' without being forcibly reminded of much 

 similar matter in Descartes's Principia. We cannot be sur- 

 prised that Descartes propounded a dynamic theory of heat, be- 

 cause his system of physical philosophy is essentially of a dyna- 

 mic character. Happily in the present day we are merging into 

 a dynamic physical system. There have been several laudable 

 attempts to precipitate this end, — among others, the quasi-Car- 

 tesian hypothesis of molecular vortices propounded by Rankine, 

 and the dynamic treatment of electrical phenomena proposed by 



* Principia, pars quarta. The Principia of Descartes was published 

 in the year 1644, consequently in Becher's nineteenth year, and twenty- 

 five years before the publication of the Physica Subterranea. 



