68 



CHEMICAL PHYSICS. 



ditions existing before the optically active substance was brought 

 between the prisms is read off on the movable disk, which is pro- 



FIG. 28. 



a b f 



fe dfe 



Lippich's polariscope. 



vided with a scale. A lamp supplies the illumination by means of 

 a flame colored yellow by sodium. 



Chemical effects of light. The well-known bleaching effect that 

 sunlight has on many dye-stuffs shows that light has the power to 

 bring about chemical changes. The art of photography is one of 

 the practical applications of this principle. Plant-life is dependent 

 on the light that reaches us from the sun. The storage of many 

 chemicals in the dark, or in colored glass bottles, is often necessary to 

 protect them from the decomposing influence of light. 



QUESTIONS. What are our views regarding the nature and propagation of 

 light? Explain reflection, refraction, and dispersion of light. What is the 

 prismatic spectrum, and how is it obtained ? Give a full explanation of the 

 spectroscope and of its use in chemical analysis. Define continuous, bright- 

 line, and absorption-spectra, and state the conditions under which they are 

 formed. What is meant by double refraction and by polarization of light? 

 Mention the essential parts of the polariscope, and the use made of it in chem- 

 ical analysis. How are the different colors produced under the influence of 

 white light? What is revealed to us by the Frauenhofer lines in the solar 

 spectrum? Explain the terms dextrorotatory, levorotatory, and optically 

 inactive substances. 



