CONSTRUCTION OF SHELLS. 7 



from the nature of the solvent. Thus if a sufficient quantity 

 of nitric acid, considerably diluted either with water or 

 spirits of wine, is poured upon a shell contained in a glass 

 vessel, it will soon exhibit a soft floating substance, consisting 

 of innumerable membranes, which retain the figure of the 

 shell, and afford a beautiful and popular object for the 

 microscope. In analysing shells of a finer texture than such 

 as are generally submitted to the test of experiment, the 

 greatest circumspection is necessary. So much so that M. 

 Herissant, whose attention was particularly devoted to the 

 subject, after placing a porcelain shell in spirits of wine, 

 added from day to day, for the space of two months, a 

 single drop of spirits of nitre, lest the air, generated or let 

 loose by the action of the nitric acid on the earthly sub- 

 stance, should tear the net-work of the fine membranaceous 

 structure. This gradual operation was attended with com- 

 plete success, and a delicate and beautifully reticulated 

 film, resembling a spider's web in texture, rewarded the 

 patience of the operator; the organization of which, from 

 its extreme fineness, he was not, however, able to delineate. 

 In shells of peculiar delicacy, even five or six months are 

 sometimes necessary for their complete development ; but, 

 in others of a coarser texture, the process is soon completed. 



