526 
ME. W. CROOKES ON EEPULSION RESULTING EEOM RADIATION. 
rature. Many persons believe that there is a peculiar emanation or aura proceeding 
from the human hand, and Baron Von Keichenbach * considered that he had proved 
this to be the case. Were this true it was not impossible that the emanation would 
affect the pith index. I have been unable, however, to detect the slightest action 
exerted by my own or any other person’s hand which I could not entirely explain by 
an action of heat. 
92. A similar series of experiments were tried with various large crystals, which were 
presented in different ways and with various precautions to the pith index. At first a 
decided action was observed ; but in proportion as precautions were taken to eliminate 
the effect of heat, so was the action seen to diminish, until very little doubt was left in 
my mind that the slight residual action would have been entirely stopped had it been 
possible, with the apparatus then used, to altogether eliminate the action of heat. 
93. Attempts were made to see if chemical action would attract or repel the index. 
I could not, however, produce chemical action close to the exhausted bulb, without at 
the same time liberating such an amount of heat as to mask any other action. 
94. Although I most frequently speak of repulsion by heat , and in illustrating any 
of the results obtained I generally use either the fingers or the flame of a spirit-lamp as 
a convenient source of radiation, it must be clearly understood that these results are not 
confined to the heating-rays of the spectrum, but that any ray, from the ultra red to the 
ultra violet, will produce repulsion in a vacuum. I have already mentioned this fact in 
my first paper (58, 68). Experiments proving the similarity of action of all rays of the 
spectrum were shown before the Physical Society on June 20th, 1874 f. They were, 
however, tried with a less perfect apparatus than the one I have since used for the same 
purpose, and need not be further alluded to till I describe the most recent results 
obtained with the spectrum (110, 111). 
95. Some experiments were tried with the object of ascertaining whether the attrac- 
tion by heat, which, commencing at the neutral point (30 et seq.), increased with the 
density of the enclosed air, would be continued in the same ratio if the apparatus were 
filled with air above the atmospheric pressure. Two bulbs containing ivory needles 
suspended by silk fibres were accordingly adjusted to show the same sensitiveness to a 
hot body. One was kept for comparison, and the other was attached to an apparatus 
whereby the internal air-pressure could be artificially increased by a column of mercury. 
A little increase of pressure was enough to show that the sensitiveness to radiation was 
greater ; and under a pressure of 14 atmosphere the superior delicacy of the ivory in the 
dense air was very marked. Attempts to carry the pressure to higher points failed, 
owing to the bursting of the thin glass bulbs. With a little different arrangement 
no difficulty would be experienced in carrying the experiments to a much higher point ; 
but hitherto the greater interest attending the vacuum experiments has prevented me 
from working further in this direction. My friend and pupil, Mr. C. H. Gimingham, 
* Researches on Magnetism &c., translated by Dr. Gregory. London, 1850. 
t Phil. Mag., August 1874. 
