OF ARTS AND SCIENCES. 379 



in making the family of Naiades conform to this view. The 

 different species of these shells are stamped with most of the 

 principal features of a natural family, and yet they have very 

 different forms ; some being nearly square, others oblong, oth- 

 ers cylindrical, others oblique, (fcc. They should agree in 

 form, or else the definition of Family which he had adopted 

 is not true. The mantle of these mollusks has not a uniform 

 margin ; being provided in different species with various 

 fringes, and approaching by its edges in places, and at two 

 points being united. The shell, by the waving outline of its 

 lines of growth, conforms to the shape of the mantle within. 

 It presents an oblong or ovate figure, on the edge of which 

 are two projections, with a depression between ; and all the 

 wide differences in the apparent shape of these shells may 

 be resolved into a greater or less degree of development of 

 these parts ; so that the Naiades do not in reality form an 

 exception to the law. 



Dr. A. A. Hayes exhibited specimens of the carbonaceous 

 deposit, which has long been known to form in retorts used 

 for decomposing coal. 



" These specimens present the order of deposition from the first 

 thin film, the lamellar coating, to the thick, compact, metallic-like 

 mass, of a shining gray color, and very sonorous when struck. Those 

 pieces which are very thin are also porous, and this character is 

 preserved when the lamina becomes considerably thickened, while the 

 final result of deposition is a compact solid of a general columnar 

 form ; the lamina being obliterated by infiltration of fine carbon and 

 final cementation. * 



" The suite of specimens is intended to illustrate the following 

 remarks. 



" It is well known that the form of carbon here presented has been 

 supposed to result from the decomposition of olefiant gas, by heat; 

 olefiant gas being one of the products of coal decomposition, under 

 certain conditions. Olefiant gas is represented by C4H4, the equiva- 

 lent being four volumes, and when it is exposed to a temperature 

 above redness, it deposits carbon in considerable quantity. If the ex- 

 posure and heat be continued, the final result is carbon, as a precipi- 

 tate, and hydrogen as a gas, free from carbon. 



