STRUCTURE OF SHELL. 551 



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 hydrochloric acid on the 

 earthy substance, should tear the net-work of the fine 

 membranaceous structure. This gradual operation was 

 attended with complete success, and a delicate and beauti- 

 fully reticulated film, resembling a spider's web in texture, 

 rewarded the patience of the operator ; the organization of 

 which film, 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. Sections of shells are usually mounted 

 in Canada balsam, or in shallow cells with glycerine. 



Mr. George Eainey pointed out the remarkable fact 

 that many of the appearances presented by the shell or 

 hard structures of animals, and which had been usually 

 referred to cell-development, are really produced by the 

 physical laws which govern the aggregation of certain crys- 

 tallizable salts when exposed to the action of vegetable and 

 animal substances in a state of solution. Mr. Eainey gives 

 a process for obtaining artificially a crystalline substance 

 which closely resembles shell in its chemical structure. 



" The chemical substances to be employed in the pro- 

 duction of the artificial calculi are, a soluble compound of 

 lime, and carbonate of potash or soda, dissolved in separate 

 portions of water; and some viscid vegetable or animal 

 substance, such as gum or albumen, mixed with each of 

 these solutions. The mechanical conditions required to 

 act in conjunction with the chemical means are, the pre- 

 sence of such a quantity of the viscid material in each 

 solution as will be sufficient to make the two solutions, 

 when mixed together, of about the same density as that of 

 the nascent carbonate of lime, and a state of perfect rest 

 of the fluid in which the decomposition is going on, so 

 that the newly-formed compound may be interfered with 



