18. Twilight and the afterglow. 



19. Light cannot pass through some objects. Opaque 



bodies. 



Experiment 17. The change from trans- 

 parent to translucent and opaque. 



20. Shadows. 



a. The cause of shadows. 



b. Variation in the shape and size of shadows. 



Experiment 18. The shape and size of 

 shadows. The shadow of the sphere. 



c. The magic of shadows. 



Experiment 19. Shadow pictures. 



d. The height of buildings and trees found by 

 their shadows. 



Experiment 20. Height from shadows. 



21. The shadow of the earth upon the moon. 



Experiment 21. A report upon an 

 eclipse of the moon. 



22. The effect of light upon bacteria. Hygienic liv- 



ing. 



(This subject, Light, is continued in the work 

 for the seventh grade.) 



HEAT. VI. 



23. A brief summary of the facts stated under the 



same topic in the work for the fifth grade. 



24. Convection. (See Section 92.) 



a. In water.- 



Experiment 22. The hot-water system 

 of heating. 



b. In air. 



Experiment 23. The hot-air system of 

 heating. 



