THE MECHANICAL ACTION OF LIGHT. 263 



me by almost every one ; and a good many appear to think that, if 

 the motion can be explained by an action of heat, all the novelty and 

 the importance of the discovery vanish. Now, this question of light 

 or heat is one I cannot answer, and I think that when I have ex- 

 plained the reason you will agree with me that it is unanswerable. 

 There is no physical difference between light and heat. Here is a 

 diagram of the visible spectrum (Fig. 5). The spectrum, as scientific 



r_ 



Fig. 5. 



men understand it, extends from an indefinite distance beyond the red 

 to an indefinite distance beyond the violet. We do not know how far 

 it would extend one way or the other if no absorbing media were 

 present; but, by what we may call a physiological accident, the 

 human eye is sensitive to a portion of the spectrum situated between 

 the line A in the red to about the line H in the violet. But this is 

 not a physical difference between the luminous and non-luminous 

 parts of the spectrum ; it is only a physiological difference. Now, the 

 part at the red end of the spectrum possesses, in the greatest degree, 

 the property of causing the sensation of warmth, and of dilating the 

 mercury in a thermometer, and of doing other things which are con- 

 veniently classed among the effects of heat ; the centre part affects 

 the eye, and is therefore called light ; while the part at the other end 

 of the spectrum has the greatest energy in producing chemical action. 

 But it must not be forgotten that any ray of the spectrum, from what- 

 ever part it is selected, will produce all these physical actions in more 

 or less degree. A ray here, at the letter C for instance in the orange, 

 if concentrated on the bulb of a thermometer, will cause the mercury 

 to dilate, and thus show the presence of heat ; if concentrated on my 

 hand I feel warmth; if I throw it on the face of a thermo-pile it will 

 produce a current of electricity ; if I throw it upon a sensitive photo- 

 graphic plate it will produce chemical action ; and if I throw it upon 

 the instrument I have just described it will produce motion. What, 

 then, am I to call that ray ? Is it light, heat, electricity, chemical 

 action, or motion ? It is neither. All these actions are inseparable 



