604 DR DAVY’S OBSERVATIONS ON FISH, 
the contrary, a free acid, the phosphoric, analogous to what occurs in the ash of 
the yolk of thé domestic fowl,—and in consequence of which, the complete incine- 
ration of the roe of the fish and its milt, like that of the yolk of the egg, is very 
difficult. 
The same conclusion, on the same ground, viz., the absence of iodine, is appli- 
cable to fresh-water fish,—a conclusion that can hardly be tested by experience, 
nor is it of practical importance, since fish of this kind enters so sparingly into 
the ordinary diet of the people. 
LesxetH How, AmuBuzsipp, 
April 14, 1853. 

P.S.—I have mentioned briefly the test employed to detect iodine. To prevent 
obscurity, may I be permitted to add a few particulars relative to the mode of 
‘proceeding? On a portion of starch in fine powder, that is, in its granular state, 
aqua regia is poured, or about equal parts of nitric and muriatic acid, in a platina 
capsule, and then well mixed, using a glassrod. The salt to be tested, either in 
solution or solid, is then added. The blue tint due to the presence of iodine is 
immediately produced, if any of this substance, or a sufficiency of it to take effect, 
be present. The delicacy of this test is, I believe, well known. I have by means 
of it detected iodine, when one-tenth of a grain of the iodide of potassium was 
dissolved in 16775 grains of water. Relative to this method, I may further remark, 
that by well mixing the acid and starch, not only is the starch reduced to a gela- 
tinous state favourable for being acted on by the iodine as liberated, by the action 
of the chlorine, but also that the excess of chlorine is, to a great extent, got rid of. 
The platina capsule has appeared preferable to one of glass, as shewing the effect 
of colour by reflected light more readily and distinctly ; and also, I am disposed 
to think, from some peculiar influence which the metal exercises, favouring the 
combination of the starch and iodine, similar, it may be, to that of spongy plati- 
num, in effecting the union of oxygen and hydrogen. 
In seeking for iodine in animal substances by incineration, it may be well to keep 
in mind, that, experimentally considered, the liability to error lies in underrating, 
rather than in overrating the result by the methods employed, and that mainly in 
consequence of more or less of loss of iodine being sustained in the process of com- 
bustion, incineration, and evaporation used. To illustrate this by a simple experi- 
ment, I may mention that a portion of water, equivalent to about 1525 grains, in 
which were dissolved 10 grains of common salt, and ‘09 grain of iodide of potas- 
sium, was quickly evaporated to dryness by boiling. Previously, the iodine could 
be detected in the mixture by the test I have used; but not afterwards, when the 
