162 
MR. W. CROOKES ON THE ILLUMINATION OF LINES OF 
FOCUS OF MOLECULAR HEAT OF IMPACT. 
581. In the experiment described at pars. 536 to 540, the heating effect of the 
molecular bombardment is assumed to be the cause of certain phenomena. It was 
thought that by concentrating the molecular impacts to one point the heat produced 
might be rendered apparent. The experiments tried in the apparatus shown in fig. 21 
prove that this supposition is correct. A polished aluminium cup a is made the nega¬ 
tive pole in a properly exhausted tube. The focus is seen very sharp and distinct, as at A, 
and of a dark blue colour. The light, although blue in the centre of the tube, when it 
spreads out and strikes the tube at the end, illuminates it beautifully with the yellowish- 
green light. By means of a magnet the focus was deflected to the side of the tube, 
as shown at B, the path of the rays being beautifully curved. On the tube the appear¬ 
ance was that of a sharply-defined oval, of a yellowish-green colour with a dark spot in 
the centre. To ascertain if heat were developed here I touched it with my finger and 
immediately raised a blister. The spot where the focus fell was nearly red hot. 
582. Another apparatus was now made, as shown in fig. 22. A nearly hemispherical 
cup of polished aluminium a is made one pole in a bulb, and a small disk of aluminium 
b is made the other pole. At c a strip of platinum is held by a wire passing through 
the glass, and forming another pole at cl. The tip of the platinum strip is brought to 
the centre of curvature, and the whole is exhausted to a very high point. On first 
turning on the induction current, the cup being made the negative pole, the platinum 
strip entered into very rapid vibration. This soon stopped, and the platinum quickly 
rose to a white heat, and would have melted had I not stopped the action of the coil. 
The • same phenomena of ignition take place if the platinum strip itself is made 
the positive pole. 
583. Experiments in a similar tube in which a piece of charcoal is the body to 
