1034 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[June 28, 1873> 
1 gram HgCl 2 is dissolved to 20 c.c. with distilled water, of 
this solution 16 c.c. is capable of removing the whole of the 
I from 1 gram KL If therefore the KI be mixed with 
KBr a proportionably less quantity of the mercuric 
solution will be required. 
It is a somewhat curious fact, and one which I have no¬ 
where seen recorded, that when exactly half the meicuiic 
solution is added a permanent precipitate begins to form, 
and I consider it highly probable that the reaction which 
takes place is analogous to that which occurs in testing 
HCN by what is termed Liebig’s process, in which a 
volumetric solution of AgN0 3 is added to a weighed 
quantity of HCN ; and when exactly one-half the cyan¬ 
ogen is displaced, and permanent precipitate of AgCN 
commences to form, a soluble double salt is first formed 
and shown by the precipitate dissolving as fast as it 
appears until exactly half the solution has been used ; the 
HCN is first made slightly alkaline with NaHO 
Ag N0 3 +2 NaCN=AgCN, NaCN+NaN0 3 
and AgCN, NaCN+AgN0 3 =2 AgCN+NaN0 3 
and similarly with KI and Hg Cl 2 
HgCl 2 +4 KI=(KI) 2 HgI 2 +2KCl 
and _ 
(KI) 2 Hg 2 I 2 +HgCl 2 =2 HgI 2 +2 KC1 
I prefer to make the formation of a permanent precipi¬ 
tate as the finishing point of the process ; it is much more 
sharply defined than the point at which the precipitate 
ceases to form. Should it be preferred, however, to make 
use of this latter (and it is useful as a check on the first 
part of the process) the best way is to take a glass tube 
open at both ends, about 15 cm. in length and 5 mm. 
diameter, and to tie over one end a small piece of filtering 
paper ; on moistening this and depressing the tube into 
the iodide solution, a few drops may be filtered off and 
transferred (by inverting the tube) to a porcelain slab and 
small test glass, and a drop of the mercuric solution 
added ; it will then be instantly apparent whether 
the reaction is complete or not. 
I append the result of my experiments with KI con¬ 
taining quantities of KBr. 
Percentage of KBr in 
1 gm KI 
Precip. 
commenced. 
Precip. ceased. 
0 
8 - 0 cc. 
16*0 cc. 
5 
7'6 cc. 
15-3 cc. 
10 
7‘3 cc. 
14 - 6 cc. 
15 
6'8 cc. 
13'6 cc. 
20 
6‘4 cc. 
12'8 cc. 
25 
6 - 0 cc. 
11'9 cc. 
[ 30 
5'7 cc. 
11-3 cc. 
1 gram KI=81 gram HgCLj 
In conclusion, I would strongly recommend pharmacists 
to practise these volumetric processes, for they afford easy 
means of determining purity in many cases in a few 
minutes where the ordinary processes take as many 
hours. 
CORAL ISLANDS AND THEIR ARCHITECTS.* 
BY PROFESSOR ALLMAN, F.R.S. 
The speaker commenced by giving an account of the 
structure and habits of the Actinia, or sea-anemone, as a 
type of the coral polype. He described it as a fleshy sac 
attached by a broad base at one end, and having at the 
opposite end a mouth surrounded by a wreath of tentacles 
or feelers. He showed that its stomach consists of a 
smaller sac suspended in the larger, and opening at one 
end by the mouth, while at the other end it opens into 
* Abstract of lecture delivered at the Royal Institution 
March 14, 1873. 
the surrounding cavity of the larger sac. This free open¬ 
ing of the stomach into the general cavity of the body is a 
character of great importance, and is possessed by no 
other group of animals than that to which the sea-anemone 
belongs. . 
The group so constituted is named Ccelentcrata, a desig¬ 
nation derived from the Greek, and intended to express 
the peculiarity of structure here referred to. This curious 
open stomach is kept in its place by a set of vertical 
fleshy plates, which radiate from it to the surrounding 
walls of the great cavity of the body. Now, the various 
parts of the sea-anemone are under the control of a defi¬ 
nite number, by which its symmetry is regulated. This 
is the number six ; and the radiating plates just men¬ 
tioned are thus either six in number or some multiple of 
six, and so also the tentacles, when freely and normally 
developed, are either six or one of the multiples. 
While the sea-anemone is immersed in the waters of 
the sea, and is surrounded by the conditions which pro¬ 
mote its health and well-being, it will display its beautiful 
crown of tentacles and spread itself out like an expanded 
flower, often brilliantly and beautifully coloured. But 
let danger threaten, let a finger rudely touch it, and it 
will instantly draw in its tentacles and contract itself 
around them; and then, instead of the expanded flower, 
we may believe we have before us the same flower in the 
bud. 
The speaker then pointed out how Caryophyllia, which 
may be found upon various parts of our own shores at¬ 
tached to rocks at low spring-tides, and which essentially 
agrees in structure with the sea-anemone, has the power 
of separating carbonate of lime from the sea-water, and of 
depositing this mineral, particle by particle, in its tissues, 
so as to become in great part calcified; in other words, 
how it has the power of forming a true skeleton of coral. 
Now the Caryophyllia, with its skeleton of coral, instead 
of remaining simple and solitary, like the sea-anemone, 
may throw out buds like a plant, or may complicate itself 
by partially dividing into two or more segments, and will 
thus be converted into a compound colony of coral-pro¬ 
ducing polypes, each polype having its own mouth and 
stomach and tentacles and radiating plates ; and each, 
while providing for its own wants, contributing at the 
same time to the general well-being of the colony. The 
form of this colony will depend on the disposition and 
mode of growth of the buds, or of the new polypes pro¬ 
duced by the splitting of the old ones; for if all these re¬ 
main closely impacted together, there would result such 
massive corals as are familiar in A stvcca and Meanch'ina / 
while if they become more or less separated from one an¬ 
other and grow out into branches, we should then have a 
branched coral like a Dendvophyllia or a Madrepore. 
Now it is such animals as these Astrseas and Meandrinas 
and Dendrophyllias and Madrepores which form the reefs 
and coral islands to whose history the discourse is devoted. 
It must not, however, be supposed that all coral ani¬ 
mals are reef-builders. The well-known red coral of com¬ 
merce, for example, never accumulates in reefs or banks. 
This coral also, as well as many other non-reef-building 
kinds, differs in some subordinate points of structure from 
the true reef-builders ; and instead of having its symmetry 
controlled by the number six, it is the number four by 
which the disposition of all its parts is regulated. 
Having thus given an account of the nature of the 
coral animal and its mode of forming coral, the speaker 
proceeded to describe the home of the reef-builders. This 
region forms an irregular zone, extending for some dis¬ 
tance at each side of the equator, but never going farther 
from it than about 30° of latitude. There can be no 
doubt that this coral zone is limited by temperature, and 
it has been pointed out by Dana, on what appears to be 
good evidence, that its northern and southern boundaries 
are formed by the isocrymal lines of 68°, or those lines 
which may be drawn through the parts of the ocean where 
the mean temperature of the coldest month of the year is 
68 ° F. These isocrymal lines, however, greatly deviate 
