218 CITROME 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 

 conld 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) '^^ 55 55 55 



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. 



55 ^' All „ 5, J5 



„ 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. 



