Mechanical Action of Light. 
[April, 
the sensation of warmth, and of dilating the mercury in a 
thermometer, and of doing other things which are conve- 
niently classed among the effects of heat; the centre part 
affedts the eye, and is therefore called light ; whilst the part at 
the other end of the spedtrum has the greatest energy in 
producing chemical action. But it must not be forgotten 
that any ray of the spedtrum, from whatever part it is 
seledted, will produce all these physical adtions 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 photographic 
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, elec- 
tricity, chemical adtion, or motion ? It is neither. All these 
adtions are inseparable attributes of the ray of that particular 
wave-length, and are not evidences of separate identities. I 
can no more split that ray up into five or six different rays, each 
having different properties, than I can split up the element 
iron, for instance, into other elements, one possessing the 
specific gravity of iron, another its magnetic properties, a 
third its chemical properties, a fourth its conducting power 
for heat, and so on. A ray of light of a definite refrangi- 
bility is one and indivisible, just as an element is, and these 
different properties of the ray are mere functions of that re- 
frangibility, and inseparable from it. Therefore when I tell 
you that a ray in the ultra red pushes the instrument with a 
force of ioo, and a ray in the most luminous part has a 
dynamic value of about half that, it must be understood 
that the latter adtion is not due to heat-rays which ac- 
company the luminous rays, but that the adtion is one purely 
due to the wave-length and the refrangibility of the ray 
employed. You now understand why it is that I cannot 
give a definite answer to the question — “ Is it heat or is 
it light that produces these movements ? ” There is 
no physical difference between heat and light, so, to avoid 
confusion, I call the total bundle of rays which come from a 
candle or the sun, radiation. 
I found, by throwing the pure rays of the spedtrum one 
after the other upon this apparatus, that I could obtain a 
very definite answer to my first question — “ What are the 
adtual rays which cause this adtion ? ” 
The apparatus was fitted up in a room specially devoted 
to it, and was protedted on all sides, except where the rays 
