1887.] Professor Dewar on Light as an Anah/tic Agent. SI 



WEEKLY EVENING MEETING, 



Friday, April 1, 1887. 



Sir Frederick Abel, C.B. D.C.L. F.E.S. Manager and Vice- 

 Presidentj in the Chair. 



Professor Dewar, M»A. F.R.S. MMJ. 



Light as an Analytic Agent. 



Analytical research by means of the agency of light is usually 

 carried out by observing the effects produced on ifght of known 

 quality by transmission through or reflection from matter; or on the 

 other hand, by stimulating matter by the agency of heat or electricity 

 to evolve light, and thereby noting its special qualities 



In previous lectures I have given an account of the results of a 

 series of experiments, undertaken in concert with my colleague Prof 

 Livemg, with the object of elucidating certain obscure speclroscopic 

 phenomena, and I propose this evening to extend the record of our 



Not long since Berthelot published the results of some investiga- 

 tions of the rate of propagation of the explosion of mixtures of oxylen 

 with hydrogen and other gases. He found that in a mixture of oxvL^en 

 and hydrogen m the proportions in which they occur in water, the ex- 

 plosion progressed along a tube at the rate of 2841 metres per second • 

 not lar from the velocity of mean square for hydrogen particles on 

 the dynamic theory of gases, at a temperature of 2000°. 



Ihis IS a velocity which, though very far short of the velocity of 

 light, bears a ratio to it which cannot be called insensible. It is in 

 lact about totVo part of it. Hence if the explosion were advancing 

 towards the eye, the waves of light would proceed from a series of 

 particles lit up m succession at this rate. This would be equivalent 

 to a shortemng of the wave-length of the light by about ^ J — part • 

 and m the case of the yellow fodium lines^ould proklJe^rshfft of 

 the lines towards the more refrangible side of the spectrum by a 

 distance of about -j-i^ of the space between the two lines. It would 

 require an instrument of very high dispersive power and sharply 

 dehned lines to make such a displacement appreciable. With lines 

 of longer wave-length than the yellow sodium lines, the displacement 



ZrA ' f ^P^^'^^^^.^*^VT ^'""^T- ^^^■*^^"' ^^ ^ receding explosion 

 could be observed simultaneously with an advancing explosion the 

 relative shift of the line would be doubled, one imlge of the line 

 observed being thrown as much towards the less-refrangible side of 

 Its proper position as the other was thrown towardi the more- 

 refrangible side. The two images of the red line of lithium would in 



G 2 



