418 Prof. F. von Kobell on Dianic Acid. 
cing flame. When the borax glass is saturated, it remains trans- 
parent on cooling after exposure to a good heat ; on being warmed 
again it becomes cloudy, and assumes the appearance of enamel. 
When the acid in question is boiled with zinc instead of tin, 
the blue solution is not obtained; the precipitate of the acid is 
blue, it is true, but the filtrate is colourless, and the acid loses its 
colour on the addition of water without being perceptibly dis- 
solved. It was only with a very large quantity of hydrochloric 
acid and zinc that I could obtain a dirty greenish solution, which, 
however, when diluted with half its amount of water, became 
speedily reduced in its colour, assuming a pale green hue with 
opalescence. 
If equal quantities of the acid spoken of, of tantalic acid, and 
of hyponiobic acid, all three bemg measured in a platinum fun- ~ 
nel, are boiled for three minutes with concentrated hydrochloric 
acid without tin in the manner above described, and are then 
poured out into a glass, they all three give yellow milky fluids. 
On the addition of a very moderate quantity of water the acid 
in question becomes perfectly transparent, whereas the tantalic 
acid, and also the hyponiobic acid, even on the addition of four — 
or five times their volume of water, remain undissolved, 
If the metallic acid im question, when freshly precipitated, is 
heated to boiling in diluted sulphuric acid (1 volume of concen- 
trated acid to 5 of water), it forms a cloudy fluid ; and on this being 
poured into a glass with a few grains of distilled zinc, im the course 
of a few minutes the acid, which was previously white, becomes 
of a decided smalt-blue, even deep blue, and retains this colour for 
some time on the addition of water; the filtrate, however, is 
colourless. In this behaviour it resembles hyponiobic acid, whereas 
tantalic acid treated in the same manner is only coloured pale 
blue, which colour immediately disappears on the addition of 
water. The difference in the behaviour of tantalic and hyponiobie 
acids has been already mentioned by HeinrichRose as character- 
istic ; as I modified the experiment, by having recourse to a boil- 
ing temperature, the effect is not only produced more rapidly, 
but also in a more marked manner. I look on this reaction for 
distinguishing tantalic acid from other kindred acids as the most 
certain, that is to say, if one does not wish to investigate the be- 
haviour of the chlorides. For a qualitative testing of an acid of 
this class, the first step of the inquiry would be to precipitate 
it in the manner described from the solution of potash, and then 
to examine the solubility of the freshly obtained precipitate with 
hydrochlorie acid and tinfoil, with due attention to the condi- 
tions above laid down. Should the acid not be dissolved to a blue 
fluid when, after three minutes’ boiling, half a cubic inch or a 
cubic inch of water is added, it is tantalic or hyponiobic acid, and 
