452 BACON 



ing other bodies by any communication of the original heat, but 

 only by exciting the parts of the heated body to that motion 

 which is the form of heat, and of which we spoke in the first 

 vintage of the nature of heat. Heat, therefore, is excited much 

 less rapidly and readily in stone or metal than in air, on account 

 of the inaptitude and sluggishness of those bodies in acquiring 

 that motion, so that it is probable, that there may be some sub- 

 stances, towards the centre of the earth, quite incapable of being 

 heated, on account of their density, which may deprive them of 

 the spirit by which the motion of excitement is usually com- 

 menced. Thus also the magnet creates in the iron a new dis- 

 position of its parts, and a conformable motion, without losing 

 any of its virtue. So the leaven of bread, yeast, rennet, and 

 some poisons, excite and invite successive and continued motion 

 in dough, beer, cheese, or the human body ; not so much from 

 the power of the exciting, as the predisposition and yielding of 

 the excited body. 



Let the thirteenth motion be that of impression, which is also 

 a species of motion of assimilation, and the most subtle of dif- 

 fusive motions. We have thought it right, however, to con- 

 sider it as a distinct species, on account of its remarkable dif- 

 ference from the two last ; for the simple motion of assimilation 

 transforms the bodies themselves, so that if you remove the first 

 agent, you diminish not the effect of those which succeed ; thus, 

 neither the first lighting of flame, nor the first conversion into 

 air, is of any importance to the flame or air next generated. 

 So, also, the motion of excitement still continues for a consider- 

 able time after the removal of the first agent, as in a heated body 

 on the removal of the original heat, in the excited iron on the 

 removal of the magnet, and in the dough on the removal of the 

 leaven. But the motion of impression, although diffusive and 

 transitive, appears, nevertheless, to depend on the first agent, 

 so that upon the removal of the latter the former immediately 

 fails and perishes ; for which reason also it takes effect in a mo- 

 ment, or at least a very short space of time. We are wont to 

 call the two former motions the motions of the generation of 

 Jupiter, because when born they continue to exist ; and the lat- 

 ter, the motion of the generation of Saturn, because it is im- 

 mediately devoured and absorbed. It may be seen in three 

 instances: i. In the rays of light; 2. in the percussions of 

 sounds ; 3. in magnetic attractions as regards communication. 

 For, on the removal of light, colors and all its other images dis- 

 appear, as on the cessation of the first percussion and the vibra- 

 tion of the body, sound soon fails, and although sounds are 

 agitated by the wind, like waves, yet it is to be observed, that 

 the same sound does not last during the whole time of the rever- 

 beration. Thus, when a bell is struck, the sound appears to be 



