220 
PROFESSOR TYNDALL ON THE ABSORPTION AND 
The plate of rock-salt was again removed, and the length of the lining was reduced 
to a foot; the dynamic radiation on the entrance of dry air in three successive 
experiments gave the deflections 
76°, 74°, 75°. 
The plate was again removed and the lining reduced to 3 inches; the deflections 
obtained in two successive experiments were 
66°, 65°. 
Finally, the lining was reduced to a ring only 1^- inch in width ; the dynamic radia- 
tion from this small surface gave in two successive trials the deflections 
56°, 56°-5. 
The lining was then entirely removed, and the deflection instantly fell to 
7°-5. 
A coating of lampblack within the tube produced the same effect as the paper lining ; 
common writing-paper was almost equally effective ; a coating of varnish also produced 
large deflections, and the mere oxidation of the interior surface of the tube is also very 
effective. 
In the above experiments the lining was first heated, and it then radiated its heat 
through a thick plate of rock-salt against the pile. The effect of the heat was enfeebled 
by distance, by reflexion from the surfaces of the salt, and by partial absorption. 
Still we see that the radiation thus weakened was competent to drive the needle almost 
through the quadrant of a circle. If instead of being thus separated from the lining 
the face of the pile itself had formed part of the interior surface of the tube, receiving 
there the direct impact of the particles of air, of course the deflections would be far 
greater than the highest of those above recorded. Indeed I do not doubt my ability to 
cause the needle of my galvanometer to whirl, by the dynamic heating of the surface of 
my pile, through an arc of 1000 degrees. Assuredly an arrangement subject to disturb- 
ances of this character cannot be suitable in experiments in which the greatest delicacy 
is necessary. 
Experiments on dynamic radiation, similar to those executed with gases, were made 
with vapours. The tube was divided into two compartments as before. Both compart- 
ments being exhausted, vapour was permitted to enter the first chamber. Dry air was 
afterwards permitted to enter the same chamber; the air was heated, it warmed the 
vapour, and the vapour radiated its heat against the pile. The heat passed in the first 
experiment through a vacuous second chamber, and in the second experiment through 
the same chamber when it contained 0’5 of an inch of the same vapour as that from 
which the rays issued. A third experiment was made to determine the dynamic radia- 
tion from the second chamber. The following Tables contain the results * — 
