244 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
INTRODUCTION. 
220. In this Part I propose to give the results of some researches which, during the 
past twelve months, have occupied much of my time. The researches include a 
quantitative examination of the repulsion exerted by a standard flame shining on pith 
and mica disks, coated with various powders, chemical precipitates, &c., and suspended 
vi vacuo in a torsion apparatus. The character of the incident radiation has been 
varied by straining it through water, alum, or coloured media ; the action of good and 
bad conductors of heat has been compared ; and the influence which favourable presen- 
tation of the experimental surface, by curvature or obliquity, has upon its movement 
has been investigated, with the result of throwing much light on some of the debated 
problems in molecular physics to which, by general assent, the repulsion resulting 
from radiation is held to be due. In every step of this investigation, theory and 
observation have gone hand in hand, and at each point gained it has been my 
endeavour to permanently record such experimental proof in the convenient form of 
an instrument, so as to have it available for further examination. 
The reaction, along lines of greatest molecular pressure, between the experimental 
surface and the fixed case containing it, has been examined. Experimental proof has 
been obtained, not only of the existence of such a reaction, but of the direction in which 
it is chiefly exerted ; and the apparatus devised during this inquiry, to put each step of 
the theory to an experimental test, has led to the construction of a modification of the 
radiometer named the otheoscope, in which the reacting surface is no longer the side of 
the glass case, but is specially made with a view of getting the greatest sensitiveness 
in the moving parts of the apparatus. Owing to the increased delicacy of the instru- 
ments now made, it has been possible to detect the existence of molecular pressure 
when radiation falls on a black surface, in air of normal density. 
MULTIPLE DISK TORSION APPARATUS. 
221. The apparatus used to get quantitative measurements of the repulsion 
produced by radiation on disks of various kinds, and coated with different substances, 
is similar in principle to the one described in Part IV. of these researches* (198, 208) ; 
but as it differs in many important details of construction it is here fully described. 
The torsion apparatus is represented in elevation in fig. 1. 
a 6 is a horizontal glass tube containing the beam, which in this case is made of 
straw, so as to secure lightness with absence of flexure under the comparatively heavy 
weights it sometimes has to bear ; glass was used at first, but it was found to bend 
too much, c d is a fine torsion fibre of glass (103), to which the beam is suspended ; 
* Phil, Trans., 1876, pt. 2, p. 365. 
