Mr. G. F. Rodwell on the Theory of Phlogiston. 25 



motion (" igneo motu"), and it is capable of communicating 

 that motion to material particles apt to receive it. The particles, 

 when endued with this rapid motion, constitute visible fire. Thus 

 it will be perceived that precisely the same kind of distinction as 

 that which Descartes admitted to exist between lux and lumen 

 was by Stahl assumed to prevail between phlogiston and ignis 

 (see p. 15). The igneous motion is " gyratorius seu verticillaris." 

 Fire is an aggregate of a great number of particles readily moved 

 by the igneous motion, and in very vehement motion ; the mo- 

 tion is circular and not progressive : " Igneus motus in seipso 

 est verticillaris, immanens in loco, non progressivus " (Exp. 81) ; 

 and again he defines it as " Verticillaris seu circa centrum 

 materialium corpusculorum " (Exp. 143)*. Fire throws out 

 different particles, varying with the nature of the substance 

 which feeds it; thus the flame of sulphur corrodes metals, while 

 that of charcoal does not affect them. Heat is an intestine 

 motion of the particles of matter. We must specially distin- 

 guish between the materia and the forma of fire, Stahl tells us : 

 the former is the thin all-pervading medium composed of very 

 moveable particles, in fact phlogiston ; the latter is the motion 

 itself. The materia is passive, and apt to receive the motion ; the 

 forma is active, and conveys the motion. In fluids each particle 

 is in a state of rapid motion ; and this is essential to fluidity. 

 Solution by a fluid only takes place when the particles of the 

 body to be dissolved possess less motion than those of the sol- 

 vent ; moreover the particles of the body to be dissolved must 

 be of two kinds, some more capable of receiving motion from 

 the particles of the solvent than others. It thus happens that 

 an inequality of motion is communicated to the particles of the 

 compound ; that is to say, some are caused to move with greater 

 velocity than others, and solution of continuity necessarily en- 

 sues. The solutive action is necessarily hastened by heat, because 

 the particles of the solvent are caused to move more quickly, 

 and consequently sooner affect the particles of the body to be 

 dissolved. 



The general application of the doctrine of phlogiston to the 

 explanation of chemical phenomena is too well known for it to 

 be necessary for me to enlarge upon it here. I may briefly state, 

 however, that as a rule the expression " loss of phlogiston " 

 means, in the language of modern chemistry, combination with 

 oxygen ; and conversely., "gain of phlogiston/ 3 or "assimilation 

 of phlogiston" signifies deoxidation. For instance, lead during 

 calcination loses phlogiston ; for lead, according to the phlogis- 

 tians, is calx of lead plus phlogiston, and when the calx is heated 



* Compare the matter of this and the preceding page with that of pages 

 14 & 15. 



