160 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



London, they were heard with great distinctness, while im- 

 mediately on the clearing away of the fog the sounds fell to 

 perhaps one fourth of their intensity. Professor Tyndall 

 concludes that the reflection and refraction of sounds, by a 

 mixture of cold and warm currents in the atmosphere, suf- 

 ficiently explains all the phenomena observed by him, and 

 this view is to a considerable extent confirmed by an experi- 

 ment devised by his assistant, Mr. Cottrell, who constructed 

 an apparatus by means of which twenty-five flat layers of 

 hydrogen are alternated with twenty-five layers of carbonic- 

 acid gas. Through this mixture of gases of different densities 

 no sound sufficient to affect the most sensitive flames was 

 able to pass. 12 A, IX., 250, 267. 



THE WIDTH OF THE SPECTRUM LINES. 



Lord Rayleigh states that in the explanation usually given 

 of the increase of w T idth of the spectrum lines w 7 ith increased 

 pressure, it appears to be assumed that their finite width de- 

 pends upon the disturbance produced by the mutual influence 

 of the colliding molecules. He demonstrates, however, that 

 the resulting lines must have a finite width, in consequence 

 of the motion of the molecules in the line of sight. If there 

 is any truth at all in the Kinetic theory of gases, the mole- 

 cules of a glowing substance are moving in all directions 

 indifferently, and with velocities whose magnitudes cluster 

 around a certain mean. The wave length of the lio-ht emit- 

 ted by the molecule, moving w T ith a mean velocity from the 

 eye, will therefore be greater by about five millionths than 

 if the molecules were at rest ; and the double of this will 

 be a moderate estimate of the width of the spectrum line ; 

 whence Rayleigh concludes that however rare the gas, and 

 however perfect our instruments may be, a fixed line can not 

 be reduced to within narrower limits than about the hun- 

 dredth part of the interval between the sodium lines. 12 ^4, 

 1873,475. 



IMPROVED FORM OF ELECTRIC LIGHT. 



A great improvement upon the old form of the electric light 

 lias lately been brought forward in Russia, and, according to 

 the journal of the Society of Arts, one that is all that is neces- 

 sary to warrant the general use of this mode of illumination, 



