1878.] 
( 283 ) 
SCIENTIFIC NOTES. 
Continuing his researches on the repulsive force resulting from radiation 
transmitted through rarefied gases, Mr. Crookes constructed a torsion balance 
in which the beam for carrying the experimental disks was a straw suspended 
by a very fine glass fibre. His experiments extended over one hundred dif- 
ferent substances, and he measured the effect produced by them, not only 
under the influence of simple radiation from a luminous source, but also by 
removing the invisible heat rays by the interposition of a water screen. In 
this manner he found that each substance exercises a more or less distinct 
action on the absorption of rays. Thus most white powders powerfully absorb 
the invisible heat rays, while they are almost without action on the luminous 
rays. On the contrary, black powders powerfully absorb the luminous rays, 
and only slightly absorb the obscure heat rays, whatever their intensity may 
be. The different metals present great differences in their action. Iron, for 
instance, chiefly absorbs the invisible heat rays, while gold is principally acted 
on by the luminous rays. The substances experimented with may be divided 
into two classes : 1st. Those whose action is increased by the interposition of 
water screens with regard to the effect produced on the standard disk; 
2nd. Those in which the contrary is the case. Amongst the former may be 
mentioned copper tungstate, saffranin, precipitated selenium, and copper 
oxalate ; these are more affected by light than by invisible heat. Amonst 
class 2 may be mentioned chromic oxide, persulphocyanogen, zinc oxide, 
barium sulphate, and calcium carbonate ; these are acted upon more by the 
ultra-red rays than by the luminous rays. Remarkable effects were also ob- 
tained by combining the substances in these two categories on the disk of the 
radiometer. Mr. Crookes has previously shown that when the exhaustion of 
a radiometer is carried beyond a certain limit its sensibility gradually dimi- 
nishes until it becomes absolutely null. He has now come to the conclusion 
— 1st. That there is a gradual increase in the sensitiveness of the radiometer 
until the pressure has attained 50 millionths of an atmosphere (0-038 millim.) ; 
2nd. That beyond this limit to 30 millionths of an atmosphere (0-013 millim.) 
it remains stationary ; 3rd. Further still, it sinks rapidly until at 1 millionth 
(0-00076 millim.) ; 4th. That at 0*2 millionth of an atmosphere (0*00015 
millim.) the radiometer refuses to turn even when five candles are put near it. 
He has also examined the effedts of molecular pressure produced diredly by 
heat upon a radiometer with the following results: — (1.) When the apparatus 
is full of air at the normal pressure of 760 m.m., and a platinum ring was 
rendered incandescent by an eledric current, the direction of the rotation of 
the vanes and disk was positive , — that is to say, that which would be produced 
by a current of air coming from the platinum ring ; this effedt must be attri- 
buted to the ascending current of hot air. (2.) At a pressure of 80 millims. 
the disk did not turn. The vanes turned slowly in the positive direction. 
(3.) At 19 millims no movement was produced either by the disk or by the 
vanes. (4.) At 14 millims. the disk remained stationary, but the vanes began 
to turn gently in the negative direction, — that is to say, in a way inverse to 
their first direction. (5.) At 1 millim. the disk turned in a continuous manner 
in the positive direction, whilst the vanes turned rather fast in a negative 
direction. (6.) At 0*224 millim* the speed of the disk or the vanes was the 
same, and their rotative movement was the same. Below that pressure the 
speed of the rotation of the vanes diminished gradually, whilst the speed of 
the disk increased, and at a pressure of 0*107 millim. the disk turned rapidly 
in the positive direction, whilst the vanes were motionless. (7.) With a more 
perfed exhaustion still, at 0*098 millim. a sudden change was seen. The 
