336 PHYSIOLOGICAL PHYSICS. [Chap. xxvi. 



greater rapidity than those of the violet, which do 

 not affect the eye, but can effect chemical changes. 

 These are called ultra-violet rays. If a beam 

 of intense white light be focussed on to a fine 

 glass bulb containing a mixture of hydrogen and 

 chlorine gases, the gases will violently unite to form 

 hydrochloric acid, and the globe will be burst with 

 a loud report. But if the beam be split up into a 

 spectrum, and the red part focussed on the globe, no 

 explosion occurs, nor yet with the yellow rays ; but 

 as soon as the violet rays fall upon the globe the 

 explosion takes place. It is the chemical activity of 

 the spectrum that permits of photography ; and photo- 

 graphy has been carried on by the agency of the 

 invisible rays beyond the violet. The spectrum, then, 

 has heating, illuminating, and chemical properties. 

 These properties are distributed throughout the whole 

 spectrum, but in different proportions, the most in- 

 tense heating effect being beyond the red end, the 

 most intense illuminating effect being in the yellow, 

 and the most intense chemical activity being beyond 

 the violet. 



These effects are all due to vibrations, 'but vibra- 

 tions of varying rates of rapidity, the rapidity in- 

 creasing from the ultra-red region, where it is least, 

 through the red to the yellow, and still increasing 

 through the violet into the ultra-violet region. The 

 vibrations of the ultra red are not rapid enough to 

 excite the retina of the eye so as to produce the sensa- 

 tion of light, while the vibrations of the ultra violet 

 are too rapid for vision. 



Fluorescence and pkosphorescence. If a 

 glass cell containing a solution of sulphate of quinine 

 be placed beyond the violet rays of the spectrum, the 

 solution becomes self-luminous, and emits a pale blue 

 light. If the spectrum be thrown on to a screen 

 which has been washed with a solution of quinine, 



