RADIOMETER. 857 



air when radiant heat is made to traverse equal lengths of column 

 of gas in each case under the ordinary atmospheric pressure ; 

 whilst the vapours of many substances liquid at ordinary tempera- 

 tures and pressures, exert even greater absorbing effects when 

 examined either under very low pressures or disseminated through 

 air (like aqueous vapour in the atmosphere). The aqueous vapour 

 itself contained in ordinary air produces so great an absorptive 

 action that the quantity present in the stratum of air lying between 

 the levels of the top of Mont Blanc and Geneva (some 14,000 feet 

 thick) is sufficient to absorb about of the radiant heat reaching 

 the earth from the sun at the former elevation ; and the difference 

 in absorptive action, according as more or less aqueous vapour is 

 present in the air of a room, is readily measurable by sufficiently 

 delicate instruments. 



Expt. 383. The Light Mill. An instrument, known as the 

 radiometer or light mill, has been constructed by Mr Crookes, con- 

 sisting of a vertical axis with horizontal wires affixed thereto, at 

 the extremities of which are vertical vanes of mica, platinum foil, 

 or other substances, clean and bright on one side and smoked on 

 the other. The whole is enclosed in a glass vacuum bulb, i.e., a 

 glass bulb from which the air is highly exhausted by means of a 

 powerful mercurial pump. The difference in the nature of the 

 front and back surfaces of each vane causes a difference in the 

 effect of the radiations received by and emitted from the two surfaces 

 respectively, the ultimate effect of which is that a mechanical force 

 is produced, causing motion of the vane. As each vane is similarly 

 situated as regards the sides blackened and not blackened, the 

 effect of each one is to produce rotation of the whole axis and 

 vanes in the same direction ; hence, on setting the instrument in 

 sucli a position that radiant energy meets it (e.g., exposing it to 

 diffused daylight or direct sunlight or electric arc light, &c.), a 

 continuous revolution is produced, the whole revolving somewhat 

 after the fashion of the sails of a windmill, whence the name 

 " light mill." 



Expt. 384. Passage of radiant heat through water. 

 Although aqueous vapour exerts a comparatively great absorptive 

 action 011 radiant heat, still liquid water is sufficiently pervious to 

 such rays to enable powerful burning glasses to be constructed by 

 filling globular vessels with water and exposing them to the sun. 

 A fishglobe or flask of distilled water can thus easily set fire to 

 gunpowder or paper, &c., placed at the focus a matter worth 

 remembering as regards the possibility of setting fire to furniture, 

 &c., since it has been proved that several serious fires have been 

 thus originated. A " bull's eye " in the glass of a window has been 



