THE MOLECULAR THEORY OF EXCITATION 6oi 



cases, where it is liable to change of sign according as the 

 stimulus is feeble or strong. Certain substances, again, 

 cannot quickly recover from the after-effect of stimulus ; 

 while, in others, recovery is fairly rapid. Recovery from 

 intense stimulation is generally, other things being equal, 

 more protracted than from feeble or moderate. Anything, 

 however, which enhances molecular freedom or mobility will 

 tend to hasten recovery. 



I shall now give several typical records in illustration 

 of the peculiarities of this form of response by resistivity 

 variation, under various conditions. The first example 



FIG. 368. Photographic Record of Response of Aluminium Powder in 

 Sluggish Condition to Stimulus of Electric Radiation 



The first two responses exhibit incomplete recovery, which becomes com- 

 plete on application of warmth. Note that warmth, increasing force 

 of recovery, hastens recovery and also diminishes amplitude of 

 response, as seen in the two succeeding records. 



given, that of aluminium powder, will be of the negative 

 type, the response being by diminution of resistance. When 

 the substance tested happens to be in a sluggish condition, 

 recovery is very protracted. There is then a response- 

 remainder of persistent negative variation, corresponding to 

 the contraction-remainder in muscle, or persistent electro- 

 motive negativity in other living tissues. But we know that 

 a moderate rise of temperature is favourable to recovery ; 

 and on applying gentle heat, at the end of the second 

 response, with its incomplete recovery, the persistent effect is 

 seen to be removed, and there is an immediate completion of 

 recovery (fig. 368). 



This is the place to refer once more to an apparent 



