218 CHROME ALUM AND CHROMIUM CHLORIDE. 



are dark greenish blue, very opaque to the visible- 

 light-rays, but transparent to the ultra-violet. I used 

 a layer ^ inch thick, which was still so dark that I 

 could not see the ants through it ; and for comparison, 

 a solution 1 inch thick of bisulphide of carbon, moving 

 them after each observation as before. 



Exp. 1. The ants were under the bisulphide of 

 carbon. 



?? ^' >j »> jj 



Exp. 3. Most of the ants were under the bisulphide 

 of carbon. 



„ 4. All but three „ • „ 



,, 5. All „ „ 



I now took chromium chloride instead of alum. 

 Exp. 1. Most were under the bisulphide of carbon. 



„ 2. All „ „ „ 



„ 3. Almost all „ „ „ 



„ 4. About three - fourths were under the 

 chromium chloride. 



„ 5. All were under the chromium chloride. 



„ 6. About two-thirds „ „ 



„ 7. About one-half under each. 



„ 8. All under the bisulphide of carbon. 



„ 9. About three-fourths under the bisulphide- 

 of carbon. 



„ 10. About half „ „ „ 



„ 11. All under the chrome alum. 



„ 12. „ bisulphide of carbon. 



