58 Observations on the Petrifactions of Shells 



taneously disengaged from tlie mud in the midst of which the shells 

 are lying. Indeed, if we scrape the black part of the shells, and 

 treat thoni with diluted chlorhydric acid, suspending a paper sa- 

 turated with acetate of lead in the phial in which the gas is dis- 

 engaged, the paper blackens in a h\v instants. 



IV. On the Chemical Composition of Shells, considered in afresh and 

 also in a petrified state, during Historical and Geological times. 



It is necessary, in order to complete the preceding observations, 

 to submit the shells petrified during the two great epochs in the his- 

 tory of the earth, to a chemical examination. 



In order to render the I'esults at which we arrive comparable, 

 we must examine the same species found petrified in geological for- 

 mations and in our own seas, for it would be illogical to compare, in 

 respect to cliemical composition, a Belemnite or an Ammonite, for 

 example, with a INIactra, a Buccinum, or any other genus of a modern 

 formation. Among the genera petrified in present times, we have 

 chosen particularly those found most frequently in that state, that is 

 to say. Oysters, Pectunculi, and Buccinum. Finally, as none of 

 these shells, Avith the exception of Ostrea, from the ocean, have 

 hitherto been subjected to analysis, we have thought it right to ana- 

 lyse chemically all these species taken in a fi'esh state. 



The following are some of the processes followed in these anslyscs. 

 The animal matter has been determined in the following manner : 

 10 grammes of the shell were taken, reduced to powder, and de- 

 prived of the water it contained by prolonged exposure to a heat of 

 about 150 degrees, till the weight of the matter ceased to undergo 

 any change. These 10 grammes of matter, perfectly free from water, 

 were then calcined to a red heat in a porcelain crucible, in order to 

 destroy the organic matter. As the read heat had necessarily de- 

 composed a part of the carbonate of lime of the shell, the calcined 

 matter was then moistened with a concentrated solution of carbonate 

 of ammonia ; a heat, below a red heat, was then carefully applied, in 

 order to recompose the cai'bonate of lime destroyed, at the expense 

 of the carbonate of ammonia. On again weighing the matter after 

 this treatment, the loss undergone by the 10 grammes of matter em- 

 ployed, represented the organic matter destroyed. We have at- 

 tempted to determine 'Jxe animal matter by another process, for the 

 difficulty of drying the shell thoroughly, without altering the animal 

 matter, may leave some doubt. The shell, dried only by a heat of 

 100 degrees, has been dissolved in hydrochloric acid, the precaution 

 being taken to add this acid in small portions, in order to prevent 

 the liquor becoming heated. The shell dissolved, leaving only the 

 insoluble animal matter under the form of filaments or delicate mem- 

 branes, just in the same manner as when bones are treated with 

 hydrochloric acid in order to extract the gelatine. 



By operating in this manner, we have always obtained a smaller 



