s g6 Mr. J. F. W. Herschel on the 
temperature. If we add an alkaline, earthy, or metallic car- 
bonate by little and little to a cold solution of peroxide of 
iron, the precipitate formed is re-dissolved with effervescence, 
readily at first, but gradually more and more slowly, till at 
length many hours, or even days, elapse before the liquid be- 
comes quite clear. Meanwhile it deepens in colour till ( un- 
less much diluted) it becomes dark brown or red. If the 
addition of the carbonate be carried as far as possible without 
producing a permanent precipitate, the solution is perfectly 
neutral, and continues clear at a low temperature for any 
length of time. In this state it may be evaporated to dry- 
ness in vacuo , and the residue (which does not effervesce with 
acids) is still soluble in water without letting any iron fall, 
and so on as often as we please. 
The compound thus formed is however far from perma- 
nent. It is in fact in a state of tottering equilibrium, which a 
very slight cause is sufficient to overset. Supposing the point 
of saturation to have been exactly attained, the addition of an 
extremely small quantity more of the alkaline solution is 
sufficient to determine the separation of the whole, or nearly 
the whole, metallic contents ; and if the solution operated on 
be pretty concentrated, it fixes after a longer or shorter time 
into a stiff and almost solid coagulum. Again, if to the co- 
agulum so formed, a quantity equally inappreciable of the 
original ferruginous solution be added, it gradually liquefies, 
and after some time is completely re-dissolved (forming no 
inapt representation of the celebrated imposture of St. Janua- 
rius's blood)* 
* The phenomenon described in the text appears to me to differ from ordinary 
precipitations and solutions, in the small proportion between the precipitant and the 
