THE 



POPULAR SCIENCE 

 MONTHLY. 



JULY, 1876. 



THE MECHANICAL ACTION OF LIGHT. 1 



By WILLIAM CROOKES, F. E. S. 



TO 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 re- 

 ceived. We heat water, and it is converted into steam, and moves 

 our machinery, our carriages, and our iron-clads. We bring a load- 

 stone near a number of iron-filings, and they move toward it, arrang- 

 ing themselves in peculiar and intricate lines ; or we bring a piece of 

 iron near a magnetic needle, and we find it turned away from its ordi- 

 nary position. We rub a piece of glass with silk, thus throwing it 

 into a state of electrical excitement, and we find that bits of paper or 

 thread fly toward 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 

 manifested in such catastrophes as those of Bremerhaven, in the 

 recent deplorable coal-mine explosions, and indeed in every discharge 

 of a gun. 



But light, in some respects the highest of the powers of Nature, 

 has not been hitherto found capable of direct conversion into motion, 

 and such an exception cannot but be regarded as a singular anomaly. 



This anomaly the researches which I am about to bring before you 



1 A lecture delivered at the Royal Institution. 

 vol. ix. 17 



