On the Erythric Acid, Be. 43 
acid very different from the erythric. Alcohol keeps dissolved a 
peculiar matter which is discovered by evaporation. If the ac- 
tion of the heat be not too strong, it leaves a colourless mass’ 
which is very tough; otherwise it isreduced toavery white spume, 
as we have seen occurring in the erythrat of potash, where the 
alkali was in excess*. This spume dissolves in water, evolving 
many bubbles, and burns with all the phenomena which accom- 
pany the combustion of animal substances. 
27. If, instead, alcohol be poured on erythrat of potash already 
altered, in this case it also becomes turbid, notwithstanding its 
acidity. The crystals which it produces are cubic, and dissolve 
in water. The solution is sweet, neutral, and, like the ‘cthers, 
presents no phenomena which could induce the belief that ery- 
thric acid is present. In such cases, the alcohol with evaporatiou 
at the fire becomes red,and is finally converted into a red spuine. 
28. The erythrat of potash reddened by heat becomes violet 
with the addition of fresh potash}: thus the red erythric acid 
takes the same colour by adding to it an excess of potash. This 
combination left in the air loses its colour and crystallizes, giving 
origin to a salt similar to that which is obtained by means of al- 
cohol. In like manner, if an alkaline solution of an alkalinule 
erythrat of potash be left to evaporate, we obtain crystalline 
groups of the same salt, that are involved in a glutinous matter. 
Erythrat of Ammonia. 
29. We recognise in this erythrat, phenomena analogous to 
those which were observed in the preceding. In adding am- 
monia to erythric acid, the union is accompanied by no sensible 
phenomenon. ‘This salt precipitates ‘with lime, vields a blue 
with solution of iron, but after a time loses this property, which 
fresh ammonia restores: exposed to the sun it reddens. 
30. Ammonia poured on crystals of erythric acid dissolves it, 
and it becomes yellow; afterwards it spontaneously grows turbid, 
deposits yellow flakes, and remains of a rose colour, transmitting 
at the same time a peculiar disagreeable odour. Those yellow 
flakes dissolve in water, and give it a rosy colour; the solution 
possesses slightly the property of colouring salts of iron, and af- 
terwards loses it. 
* It is remarkable that the solid erythrat of potash, which often sponta- 
neously reddens, yields a neutral solution, which does not redden on ex- 
posure to heat, but also produces this very white spume. 
+ The addition of ammonia to red erythrat of potash produced a very sur- 
prising phenomenon; it developed a disagreeable odour, and immediately, 
or after some time, yielded a black powder mixed with a substance which ex- 
posed to the light of the sun presented the beautiful green colour of the 
emerald. This singular change sometimes did not succeed, for which I can 
assign no reason, | 
Erythrat 
