SENSITIVE FLAMES, ETC. 187 



Molecular vibrations are, however, unable to be trans- 

 mitted by the coarser matter around us. A medium of 

 higher elasticity and of less density is necessary for the 

 transmission of the vibrations of heat and light. Such 

 a medium has been assumed to exist, and we have 

 every reason to believe that it does exist ; it is termed 

 "the ether." As you have had, in preceding lectures 

 by Professor Forbes, some fuller reference to this subject, 

 I need not dwell upon it here ; and therefore we will pass 

 at once to the manner in which vibratory energies are 

 accepted. We have seen how they are produced ; we have 

 seen that they are transmitted ; and now we have to exa- 

 mine how they are accepted. 



And here we meet at once with a very important law. 

 This law is stated as follows : The receptivity of a body 

 for vibratory energy depends on the capability of that body 

 to vibrate in periods corresponding to the rate of vibration 

 of the source. We may term this power of receptivity 

 sympathetic vibration, or the sympathetic state, and this 

 we must examine now in greater detail. Here I have a 

 pendulum which is capable of swinging to and fro in a 

 definite time. If now I bring that pendulum to rest, and 

 then blow it with my breath, I can set it in rapid vibra- 

 tion by properly timing the impulses which I give to it. 

 By the side of this pendulum is another, which is capable 

 of vibrating at precisely the same rate. If I bring one 

 of these pendulums to rest, and set the other in motion 

 by its side, it is evident that the to-and fro motion of 

 the pendulum will produce motions in the air and wooden 

 framework which will be precisely timed to the motion 

 of the first pendulum (Fig. 2). Hence, after a certain time, 

 we shall find that the to-and fro motion of the one pen- 

 dulum will set the other into vibration. If, however, 

 I shorten one of these pendulums, so as to make its rate 

 of vibration not coincident with that of the other one, then 

 the motions will not be so timed, and the second pendulum 

 will not be thrown into motion. You may have illustra- 

 tions of this transference of motion in various other ways. 

 Here, for example, I have two strings which are tuned in 

 perfect unison, and attached to the same soundboard. If 

 now I put this first string into motion, it will com- 

 municate its vibration through the wood to the second 



