1 876.] 
Mechanical Action of Light. 
231 
It is not at first sight obvious how ice or a cold body can 
produce the opposite effect to heat. The law of exchanges, 
however, explains this perfectly. The pith bar and the 
whole of the surrounding bodies are incessantly exchanging 
heat-rays ; and under ordinary circumstances the income 
and expenditure of heat are in equilibrium. Let me draw 
your attention to the diagram (Fig. 2) illustrating what takes 
place when I bring a piece of ice near the apparatus. The 
centre circle represents my piece of pith ; the arrows show 
the influx and efflux of heat. A piece of ice brought near 
cuts off the influx of heat from one side, and therefore 
allows an excess of heat to fall on the pith from the opposite 
side. Attraction by a cold body is therefore seen to be only 
repulsion by the radiation from the opposite side of the 
room. 
Fvg 2 . 
The later developments of this research have demanded 
the utmost refinement of apparatus. Everything has to be 
conducted in glass vessels, and these must be blown to- 
gether till they make one piece, for none but fused joints are 
admissible. In an investigation depending for its successful 
prosecution on manipulative dexterity, I have been fortunate 
in having the assistance of my friend Mr. Charles Gimingham. 
All the apparatus you see before you are the fruits of 
his skilful manipulation, and I now want to draw your 
attention to what I think is a masterpiece of glass-working 
— the pump which enables me so readily to produce a 
vacuum unattainable by ordinary means. 
The pump here at work is a modification of the Sprengel 
pump, but it contains two or three valuable improvements. 
I cannot attempt to describe the whole of the arrange- 
ments, but I will rapidly run over them as illuminated by 
the eleCtric light. It has a triple fall tube in which the 
