44 On the Erythrie Acid, Bc. 
Erythrat of Iron. 
Sl. Erythric acid combined with iron presents so numerous 
and variable phenomena, that te explain them would require 
exclusively a long study. Boiling erythric acid over iron filings, 
the metal dissolves, and the solution varies in colour according to 
the concentration of the acid, and the action more or less strong 
of the fire. Thus sometimes it is yellow, sometimes purple, and 
sometimes of a most beautiful blue colour. The latter however 
it acquires in every case, by means of adding an alkali, which 
does not produce any other precipitate. 
_ 82. Similar combinations are obtained by boiling erythric acid 
over black oxide of iron. It seems, however, that in such a case 
we canuot immediately obtain the blue colour, but always if it 
had the citron yellow. ‘To have that colour, the addition of an 
alkali is also necessary: after some time, indeed, the blue dis- 
appears, and a colour similar to the first returns. pia 
33. Erythrat of iron concentrated by heat leaves green bands 
and deposits yellow grains, which with slow evaporation may 
also be crystallized in close prisms. It is worthy of remark, 
that in this erythrat,what the heat of the fire cannot do, that of 
the sun can ; that is, communicate to it the blue colour. It is 
however fugitive, the yellow colour returns, and is ready to again 
become blue if exposed to alkali or the sun, and in these changes 
only a black powder is seen to be deposited. Erythrat of iron 
long exposed to the action of the sun is entirely in blackish 
natter, from which water is tinged red. 
34. Erythric acid unites even cold with peroxide of iron: this 
solution has a yellow colour : if it be deep, alkali in a small dose 
produces a turbid coagulation, which on the addition of alkali 
_dissolves and becomes blue. This coagulated matter, if left a 
long time quiet, spontaneously dissolves, and a yellowish matter 
reappears. 
35. The erythrats of iron exposed to the electric current at 
the negative pole, cover the platina wire with a blue crust which 
afterwards tinges the whole liquid. This singular fact unites 
these phenomena with those which we have before observed in 
the simple erythric acid, and demonstrates that the various co- 
lourings to which this acid is subject, depend on a common cause 
modified by the bodies with which it is found in contact. 
36. These experiments, although very incomplete, seem to 
me sufficient to show, that the blue colour which often accom- 
panies the erythrats of iron is not a property of these, but most 
likely it belongs to a substance generated in the act in which the 
erythric acid changes into that other acid of which we have be- 
fore spoken. In the passage of one acid to another On gaaS 
i 
