228 Mechanical Action of Light. [April, 
Mivart. If such are genuine “ Lessons from Nature ” the 
venerable dame keeps a most inefficient school, and the 
sooner it is closed the better. But in this case the pure 
white light has passed through such a powerfully distorting 
medium that its true nature can scarcely be recognised. 
Metaphysics may be regarded as a disease which thinkers 
are liable to contract at some part of their career, just as 
children take the measles, or rather the scarlet fever. Mr. 
Mivart’s is a very bad case ; but for his own sake, and still 
more for that of Science, we wish him a full and a speedy 
recovery. An intellect like his is too valuable to be lost. 
V. THE MECHANICAL ACTION OF LIGHT.* 
By W. Crookes, F.R.S. 
f O generate motion has been found a characteristic 
common, with one exception, to all the phases of 
Physical Force. We hold the bulb of a thermometer 
in our hands, and the mercury expands in bulk, and, rising 
along the scale, indicates the increase of heat it has 
received. We heat water, and it is converted into steam, and 
moves our machinery, our carriages, and our ironclads. 
We bring a loadstone near a number of iron filings, and 
they move towards it, arranging themselves in peculiar and 
intricate lines ; or we bring a piece of iron near a mag- 
netic needle, and we find it turned away from its ordinary 
position. We rub a piece of glass with silk, thus throwing it 
into a state of elecftrical excitement, and we find that bits 
of paper or thread fly towards it, and are, in a few moments, 
repelled again. If we remove the supports from a mass of 
matter it falls, the influence of gravitation being here most 
plainly expressed in motion, as shown in clocks and water- 
mills. If we fix pieces of paper upon a stretched string, 
and then sound a musical note near it, we find certain of 
the papers projected from their places. Latterly the so- 
called “ sensitive flames,” which are violently agitated by 
certain musical notes, have become well known as instances 
of the conversion of sound into motion. How readily 
chemical force undergoes the same transformation is mani- 
fested in such catastrophes as those of Bremerhaven, in the 
* A Le&ure delivered at the Royal Institution, on Friday evening, February 
nth, 1876. 
