40 On the Erythric Acid, Be. 
platina wire at the positive pole became of a yellow colour, and 
that at the negative was almost covered with a crust of red co- 
louring matter. | 
12. The experiment was afterwards reversed, that is to say, 
a solution of erythric acid, reddened either by the pile or by heat, 
was placed at the positive pole, and pure water at the negative, 
arranged as before. The development of gas appeared from 
both poles. After.a longer time than that which it had employed 
to redden, the liquor began to diminish in colour, and finally 
became, as at first, colourless. It was pleasing to see on the 
amianth a light rosy tint which terminated in a beautiful little 
red ring: it was insensibly moved towards the negative pole, and 
the tumbler of the positive pole was also marked by red rings or 
bands at the part towards the other pole and near the amianth. 
Changing the position of the tumblers, putting to the positive 
water, and to the negative acid without colour, the latter red- 
dened and the colour vanished on the amianth. 
13. These experiments seem to prove that the change in the 
red colouring matter of the erythric acid proceeded from the 
loss of oxygen which the acid sustained. Indeed at the negative 
pole, where it reddens, is the precise point where the developing 
hydrogen can subtract from it this principle ; and from the posi- 
tive pole the colouring matter is carried in the state of alkali to 
the other pole. 
14. I have found another proof which confirt ms the opinion 
that the appearance of the red colour in erythric acid depends 
on the cause here assigned. Immersing red-hot iron nails in 
this acid, the red colour is immediately seen to appear. I also 
hoped to obtain a similar change with phosphorus. I put a small 
piece in erythric acid, and left it at the light of the sun; in the 
fluid no notable colour appeared, and the phosphorus only ac- 
quired a violet hue. Neither was the action of fire fit in this 
case to make the fluid become red. 
Erythrats of Lime and Barytes. 
15. Erythric acid poured into lime-water, as before observed, 
makes it very turbid; with the addition however of fresh erythric 
acid it is dissolved, but not with that of any other acid although 
weak, not even the carbonic acid, which is capable of decom- 
posing the erythrat of lime, This salt is found in the form of 
light, white flakes, which are seen suspended in the fluid, and 
even rise to the surface if any extraneous substance is found 
in it. 
16. This erythrat of lime has scarcely remained any time in 
contact with the air, when it experiences a change. It is found 
(vat at its expense a carbonat is formed, judging from the vivid 
effervescence 
