Cornish and Kendall—Calcareous Organisms. 67 
Three circumstances were noted, viz. : 
1. That the calcite shell lost in weight through solution of its 
substance, but retained its compact texture and translucency, so that 
no alteration in appearance could be observed till it had lost a large 
percentage of its substance. 
2. That the aragonite shell lost by solution between two and 
three times as great a percentage of its weight as was lost by the 
calcite shell. 
3. That the hard compact surface of the aragonite shell speedily 
disappeared, the shell assuming a consistency similar to that of 
kaolin, and falling into fragments after losing 60 per cent. of its 
weight by solution. 
Experiments with other aragonite shells showed that after a short 
period of subjection to carbonic acid solution they are reduced to a 
consistency such that the substance of the shell comes away with a 
touch, and such that a gentle stream of water is sufficient entirely 
to disintegrate the shell. Calcite shells, on the other hand, after 
being acted upon lose only a small quantity of substance by washing. 
In the Coralline Crag we have observed them in a pulverulent 
state only in those portions from which the aragonite shells have 
been entirely removed. 
From observations 2 and 3 it follows that carbonated water acts 
so as to decompose aragonite fossils more readily than those formed 
of calcite, and from 3 it further follows that where water is free to 
circulate, the aragonite fossils already acted upon lose their coherence 
and are reduced to the condition of a powder. 
In addition we see from 1 that the retention of their compact 
structure and translucency by calcite fossils lends to them an 
appearance of immunity from the action of carbonic acid which they 
do not in reality enjoy. 
It remained to investigate the cause or causes of observation 2, 
and to ascertain whether the fact observed, viz. that aragonite shells 
are dissolved far more rapidly than the calcite, is due (a) to difference 
of mineralogical constitution; (b) to difference of structure of the 
shell related to that difference of mineralogical constitution ; or (c) 
to both causes combined. 
To test the first point we placed pure crystalline calcite and 
aragonite in fine powder in two flasks of the same capacity and 
shape together with equal volumes of carbonic acid solution of 
equal strength, and determined the loss of weight suffered by the 
substances after the lapse of an equal interval of time in each case. 
The result of the experiment showed no excess of action on the 
aragonite over that on the calcite. 
A similar experiment where finely powdered fossil calcite and 
aragonite shells were employed gave similar results. 
The conditions under which these determinations were made were 
not such as to eliminate certain possible sources of error, but the 
degree of accuracy obtainable is sufficient to justify us in concluding 
from the numbers given below that the more rapid solution of 
aragonite fossil shells is not due directly to difference of miner- 
alogical constitution, but to difference of structure. 
