258 THE POPULAR SCIEXCE MONTHLY. 



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 hot 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 obvi- 

 ous explanations were evidently not the true ones; obvious explana- 

 tions seldom are true ones, for simplicity is not a characteristic of 

 Nature. 



An unknown disturbing cause was interfering, and the endeavor 

 to find the clew 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, while occasionally no movement whatever was produced. 



I will try to reproduce these phenomena in this apparatus (Fig. 1). 

 Here are two glass bulbs, each containing a bar of pith about three 

 inches long and half an inch thick, suspended horizontally by a 

 long fibre of cocoon silk. I bring a hot glass rod, or a candle, toward 

 one of them, and you see that the pith is gradually attracted, follow- 

 ing the candle as I move it round the bulb. That seems a very defi- 

 nite fact ; 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 re- 

 pelled 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, and it is attracted, and 

 follows the rod round as a magnetic needle follows a piece of iron. 



The repulsion by radiation is the key-note of these researches. 

 The movement of a small bar of pith is not very distinct, except to 

 those near, and I wish to make this repulsion evident to all. I have 

 therefore arranged a piece of apparatus by which it can be seen by all 

 present. I will, by means of the electric light, project an image of a 

 pendulum suspended in vacuo on the screen. You see that the ap- 

 proach of a candle gives the bob a veritable push, and, by alternately 

 obscuring and uncovering the light, I can make the pendulum beat 

 time to my movements. 



