68 



KNOWLEDGE. 



[April 1, 1891. 



mmeralot,nst in c;illiii,u lum-^u«iie ami marble ealcite ? We 

 liave said that whore crystalline symmetry differs, all 

 physical properties are different. One of the most im- 

 portant, and one of the most nearly constant properties of 

 every kind of matter, is its specific gravity. Every 

 specimen of ealcite has a specific gravity of 2-72, or not 

 differing from 2 72 by more than one or two units in the 

 second decimal place. Similarly all specimens of ara- 

 gonite have a specific gravity of very nearly 2-93. In 

 limestone and marble we have the so-called maxsire car- 

 bonate of lime, the material not having had the oppor- 

 tunity to assume the crystalline form. How are we to 

 determine whether limestone is ealcite, aragonite, or yet 

 another mineral species ? The specific gravity is found 

 to be that of ealcite, Niz. about 2-7, as is also the case 

 with marble. We conclude, therefore, that the carbonate 

 of lime in limestone and marble is ealcite. 



It has long been a familiar fact that certain organisms 

 have the power of secreting carbonate of lime from 

 solution to build up the hard xjortion of their shells. The 

 shell of the oyster, for instance, of the crab, and of the 

 common whelk are complex structures of organic matter 

 and carbonate of lime, and in the fossilized remains of 

 shells the general form of the shell is preserved by the 

 carbonate of lime after the decomposition of the organic 

 matter. Several questions of interest are suggested by 

 this case of combined mineral and animal growth. Does 

 the carbonate of lime in calcareous organisms exist in 

 some special modification different from those known in 

 the mineral world ? How far is the mode of growth of 

 the animal tissues constrained by the rigid laws of crystal- 

 lization •? Or, on the other hand, do the forces brought 

 into play in animal growth mask, or even overpower, 

 the operation of the ordinary process of the crystallization 

 of a substance from solution? The answers to these 

 questions are furnished by the investigations of Dr. 

 Sorbey and others, which have been published at intervals 

 during the last twelve years. In the first place, it has 

 been shown that in calcareous organisms we have not 

 to deal with any new species of carbonate of lime. In 

 every case the optical properties and the specific gravity 

 show that the shells contain either ealcite or aragonite. 

 Some animal species secrete carbonate of lime as ealcite, 

 others as aragonite. 



The mode of growth of the shell is in general a com- 

 promise between the mnieral and the organic, in some 

 cases the influence of the first factor having the pre- 

 dominance, in others that of the second. The direction 

 of the primiiuit axis of the crystal is always related in a 

 definite manner to the suriace of growth of iihe shell — 

 the symmetrical arrangements which result from this 

 relation producing very beautiful appearances when sections 

 are examined under the microscope. Thus the inner shell 

 of xfiiia (the cuttle-fish) shows innumerable crystals of 

 aragonite ranged in parallel rows, whilst the mineral 

 portion of the spines of ccJiiimlenDf: consists of a single 

 crystal of ealcite greatly developed in one direction. 



Some organisms, as we have said, secrete or produce 

 ealcite, others aragonite. Other cases again are known in 

 which one portion of the shell is built up of, say, arago- 

 nite, and during the subsequent growth of the organism 

 its habits or powers undergo a change, the rest of the shell 

 being built up of ealcite. It will be seen that we are 

 dealing with a mixed study, at once mineralogical and 

 biological. It is peculiarly interesting to find that the 

 influence of the laws of evolution is apparent even when 

 studying the mineralogical aspect of the subject. It is 

 well known that the embryo shows the past history of the 

 species, the development of the individual furnishing an 



epitome of the history of development in the race. Now 

 the shell of the common whelk, which may be found on 

 any sea-beach, is composed of ealcite, except a small 

 portion which is that first formed in the growth of the 

 organism. The examination of the /ami species of the 

 whelk tribe shows that they are comjjosed wholly of arago- 

 nite, the composition of the whole shell of the early indi- 

 \iduals of the race being identical with that of the embryo 

 of their modern representative. 



It was long since observed that in fossiliferous beds 

 which are permeable to water, certain calcareous shells are 

 preserved, whilst others are only represented by their im- 

 pressions. This is the more remarkable, because some of 

 the most massive sheila have disappeared whilst others oi 

 delicate structm-e remain. The form of the impression, 

 enables the species of the shell to be identified, and it is 

 found that the shells which have been removed are those 

 of species known to secrete their carbonate of lime as 

 aragonite. It was known that under many conditions 

 aragonite is less stable than ealcite, and it was assumed 

 from the above observations that aragonite is more readily 

 dissolved than ealcite by water containing carbonic acid. 

 A few years ago this point was made the subject of expe- 

 rimental investigation, with somewhat singular results. 

 Pure and well crystallized specimens of ealcite and arago- 

 nite were subjected to the action of a solution of carbonic 

 acid under similar conditions. No difference of solubility 

 was detected. Powdered ealcite and aragonite fossil shells 

 gave a like result. It was found, however, when the com- 

 plete shoUs were suspended in a solution of carbonic acid, 

 that those of aragonite were much more readily acted upon 

 than those of ealcite, and further that the coherence of 

 the shells was soon destroyed, so that the slightest agita- 

 tion of the water was sufficient entirely to disintegrate the 

 shell, reducing it to the condition of a fine powder or mud. 

 The disappearance of aragonite fossils is explained by 

 these experiments ; it is due not to the greater solubility 

 of aragonite, but to a mode of structure of shells composed 

 of aragonite which facilitates their solution and disin- 

 tegration. 



The experiments gave somewhat unexpected results also 

 in the case of ealcite fossils. It was found that they were 

 acted upon with considerable rapidity by the solution of 

 carbonic acid, but that they retained their compactness, 

 and even the delicate details of marking, after losing as 

 much as 15 per cent, of their weight through the action of 

 the solvent. It is thus evident that the ealcite fossils 

 found in porous or iJermeable beds simulate an immunity 

 from the action of carbonic acid which they do not in 

 reality enjoy. In such beds as these a large quantity of 

 carbonate of lime goes into solution, and cavities are 

 formed having the shape of the fossils which have been 

 removed. It frequently happens that at a subsequent 

 period carbonate of lime crystallizes out from solution in 

 these cavities. When crystallization takes place at the 

 ordinary temperature ealcite is formed, a fact which is 

 readily established by laboratory experiments. Conse- 

 quently casts are formed in ealcite of aragonite fossils. 

 They are readily distinguished from the originals by their 

 translucency, aragonite fossils being always opaque. Thus 

 the beautiful ainmunitfs often found of a material resembling 

 fine alabaster are repliquas only, no trace of the original 

 aragonite shell remaining. Many other such cases occur. 

 In conclusion, we may point out that the investigation 

 of the subjects dealt with in this article involves the 

 application of chemistry, mineralogy, and pahvontology. 

 The work has been taken up from time to time by students 

 of one or other of these sciences, but the subject as a whole 

 belongs to a sort of no-man's land in science. The border- 



