242 BERTHA'S VISIT TO HER 



kind as to say it did not signify, as he had some 

 more ; but I determined in future to see without 

 the help of jny fingers, as you have often advised 

 me, dear mamma. The second piece was harder 

 and more opaque, having only a little trans- 

 parency at the edges. The third specimen was 

 perfectly opaque, and looked like a bit of stucco ; 

 it was on this piece that he shewed us the curious 

 experiments I have mentioned. 



Mr. R. endeavoured to make us comprehend 

 the causes of all those changes which have ap- 

 peared so contradictory. " In the first place," 

 said he, " tabasheer is a remarkably porous sub- 

 stance ; now if one of the pores be filled with 

 air, a ray of light in passing through it suffers 

 very little refraction, and is therefore so little 

 scattered, that the tabasheer appears transpa- 

 rent, and objects can be partly seen through it. 

 This arises from the small difference between 

 the refractive power of air and of tabasheer. 

 Next," said he, "suppose a very small quantity 

 of water introduced into this pore, so as not to 

 fill it, but merely to line it with a film ; then the 

 light, in passing through the tabasheer, the film 

 of water, the air within, the film again, and lastly 

 the tabasheer, is so much scattered by these six 

 refractions, that the substance appears opaque. 

 But when by complete immersion the pores 

 are filled with water, the multiplied refractions 

 caused by the films and the portions of air 



