1875.] ( 337 ) 
VI. THE MECHANICAL ACTION OF LIGHT. 
By WILLIAM CROOKES, F.R.S., &c. 
OME experiments illustrating the mechanical action of 
“Sy Light, which I have recently exhibited before the 
Fellows of the Royal Society, having attracted consi- 
derable attention, I propose to give here a description of 
some of the instruments which my researches have enabled 
me to construct. But, to render the subject more intelli- 
gible, it will be necessary to give a brief outline of the 
researches which I have been carrying on for the last three 
or four years, so that the reader may see the gradual steps 
which have led up to the full proof that Radiation is a motive 
power. 
The experiments were first suggested by some observa- 
tions made when weighing heavy pieces of glass apparatus 
in a chemical balance, enclosed in an iron case from which 
the air could be exhausted. When the substance weighed 
was of a temperature higher than that of the sanroundine air 
and the weights, there appeared to be a variation of the force 
of gravitation. Experiments were thereupon instituted to 
render the action more sensible and to eliminate sources of 
ecror.* 
My first experiments were performed with apparatus made 
on the principle of the balance. An exceedingly fine and 
light arm was delicately suspended in a glass tube bya double- 
pointed needle; and at the ends were affixed balls of various 
materials. Amongst the substances thus experimented on 
I may mention pith, glass, charcoal, wood, ivory, cork, sele- 
nium, platinum, silver, aluminium, magnesium, and various 
other metals. 
The most delicate apparatus for general experiment was 
made with a straw beam having pith masses at the end. 
The general appearance of the apparatus is shown in 
Bic. 1. 
A is the tube belonging to the Sprengel pump.t B is the 
desiccator, full of glass beads moistened with sulphuric acid. 
c is the tube containing the straw balance with pith ends: 
it is drawn out to a contracted neck at the end connected 
* «On the Atomic Weight of Thallium,’ Phil. Trans., 1873, vol. clxiii., 
p- 287. 
+ For a full description of this pump, with diagrams, see Phil. Trans., 1873, 
vol. claiil., p. 295. 
