1876.] Mechanical Action of Light. 229 
recent deplorable coal-mine explosions, and indeed in every 
discharge of a gun. 
But light, in some respeCts the highest of the powers of 
Nature, has not been hitherto found capable of direcSt con- 
version into motion, and such an exception cannot but be 
regarded as a singular anomaly. 
This anomaly the researches which I am about to bring 
before you have now removed ; and, like the other forms of 
force, Light is found to be capable of direCt conversion into 
motion, and of being— -like heat, electricity, magnetism, 
sound, gravitation, and chemical aCtion— most delicately 
and accurately measured by the amount of motion thus 
produced. 
My research arose from the study of an anomaly. 
It is well known to scientific men that bodies appear to 
weigh less when they are hot than when they are cold; 
the explanation given being that the ascending currents of 
Jiot air buoy up the body, so to speak. Wishing to get rid 
of this and other interfering actions of the air during a 
research on the atomic weight of thallium, I had a balance 
constructed in which I could weigh in a vacuum. I still, 
indeed, found my apparatus less heavy when hot than when 
cold. The obvious explanations were evidently not the true 
ones : obvious explanations seldom are true ones, for simpli- 
city is not a characteristic of Nature. 
An unknown disturbing cause was interfering, and the 
endeavour to find the clue to the apparent anomaly has led 
to the discovery of the mechanical aCtion of light. 
I was long troubled by the apparent lawlessness of the 
actions I obtained. By gradually increasing the delicacy of 
my apparatus I could easily get certain results of motion 
when hot bodies were brought near them, but sometimes it 
was one of attraction, at others of repulsion, whilst occa- 
sionally no movement whatever was produced. 
I will try to reproduce these phenomena in this apparatus 
(Fig. 1). Hereare two glass bulbs, each containing a barof pith 
about3 inches long andean inch thick, suspended horizontally 
by a long fibre of cocoon silk. I bring a hot glass rod, or a candle, 
towards one of them, and you see that the pith is gradually 
attracted, following the candle as I move it round the bulb. 
That seems a very definite laCt ; but look at the aCtion in 
the other bulb. I bring the candle, or a hot glass rod, near 
the other bar of pith, and it is strongly repelled by it — much 
more strongly than -it was attracted in the first instance. 
Here, again, is a third faCt. I bring a piece of ice near 
the pith bar which has just been repelled by the hot rod, 
