582 TRANSACTIONS OF SECTION 0. 



The nacre was then powdered into the finest fragments and reintroduced. The 

 powder settled to a lower position almost corresponding with aragonite. A piece 

 of nacre from a specimen not preserved in spirit was similarly treated and proved 

 to be of the same density as aragonite. 



On gently heating a flake of nacre it was found to curl up, thus indicating that 

 we are not dealing with a homogeneous substance. 



On dissolving nacre and pearl in weak hydrochloric acid, an organic residue 

 was left in such quantities as to form a spongy cast of the original. 



The columnar part of the pearl-oyster gave similar results, and this came to 

 rest in the diffusion-column just above calcite. 



The included organic matter may be a sufficient explanation of the fact that 

 the substances examined did not take up positions exactly corresponding with the 

 indices. 



This is all the more likely when we find that on powdering the substances and 

 thus separating the carbonate of lime we get results very closely approximating to 

 calcite or aragonite. 



On slicing the pearl-oyster shell and examining under the microscope, it is 

 found that the nacreous layer itself is not uniform. Patches of the columnar part are 

 occasionally seen enveloped in the platy layers of the nacre. 



Fine sections of pearls failed to show any enclosures of this nature. 



Meigen^s Test. — Two pearls and several pieces of pearl-oyster were boiled in 

 dilute cobalt nitrate, and the characteristic purple staining was produced in the 

 pearls and nacre, but not in the columnar layer. 



These experiments show that fresh pearls are composed of aragonite mixed 

 with organic matter, while the pearl-oyster shell consists of a columnar layer 

 which is calcite, and a nacreous layer which is aragonite, 



