548 NOVUM ORGANUM. [bOOK II, 



the human hody ; 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 consider 

 it as a distinct species, on account of its remarkable difference 

 from the two last ; for the simple motion of assimilation trans- 

 forms 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, are 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 imme- 

 diately fails and perishes ; for which reason also it takes effect 

 in a moment, or at least a very short space of time. We are 

 wont to call the two former motions the motions of the genera- 

 tion of Jupiter, because when born they continue to exist ; 

 and the latter, the motion of the generation of Saturn, because 

 it is immediately devoured and absorbed. It may be seen 

 in three instances : 1. In the rays of light ; 2. in the percus« 

 sions of sounds ; 3. in magnetic attractions as regards commu- 

 nication. For, on the removal of light, colours and all its 

 other images disappear, as on the cessation of the first percussion 

 and the vibration of the body, sound soon fails, and although 

 sounds are agitated by the wind, like waves, yet it is to be ob- 

 served, that the same sound does not last during the whole time of 

 the reverberation. Thus, at hen a bell is struck, the sound appears 

 to be continued for a considerable time, and one might easily be 

 led into the mistake of supposing it to float and remain in the 

 air during the whole time, which is most erroneous." For the 

 reverberation is not one identical sound, but the repetition of 

 Bounds, which is made manifest by stopping and confining the 



" Aristotle's doctrine, that sound takes place wheu bodies strike the 

 Air, which the modern science of acoustics has completely established, 

 was rejected by Bacon in a treatise upon the same subject : " The collision 

 or thrusting of air," he says, " which they will have to be the cause ot 

 sound, neither denotes the form nor the latent process of sound, but is 

 a term of ignorance and of superficial contemplation." To get out of 

 the difficulty, he betook himself to his theory of spirits, a species of 

 phenomena which he constantly introduces to give himself the air oi 

 explaining things he could not understand, or would f '^ admit upon 

 the hypothesis of his opponents. £d. 



