-¥822.] in distinguishing several Acids, &c. 5 
which is ‘always the case when it is not purified by repeated 
crystallization, it immediately occasions a very distinct yellowish 
colour, which soon disappears. The boracic acid of the Isle of 
Volcano acts very distinctly like pure boracie acid. 
Concentrated phosphoric acid gives a rose colour, by absorb- 
ing moisture from the air, it is slowly changed to orange colour. 
The acid, when diluted with 10 to 30 parts of water, gives in 
half a minute a very fine yellow colour, which remains without 
any alteration. 
Phosphatic acid cannot be distinguished by its action from 
phosphoric acid. 
Concentrated phosphorous acid gives a rose colour, which 
becomes of a yellow colour sooner than either of the last men-- 
tioned acids, and it resembles the colour produced by those 
acids when diluted. Dilute phosphatic (phosphorous?) acid 
gives a fine yellow colour, which soon becomes pale. 
~ Concentrated hypophosphorous acid gives also a red colour,, 
which becomes gradually pale-yellow, and eventually almost 
colourless; when diluted with water it gives at first a yellow 
colour nearly as fine as that of the three last named acids, but it 
soon disappears, and there remains an indistinct colour, which 
is neither red nor yellow. 
Concentrated arsenic acid produces a rose colour which 
remains for along time. Diluted with 10 to 30 parts of water, it 
gives in one minute a very fine yellow colour, but in a few 
minutes, it fades, and becomes and remains pale-yellow. 
Arsenious acid has no marked action. 
Concentrated acetic acid gives a dull-yellow colour directly, 
which disappears immediately, and is succeeded by a pale- 
violet colour, which, by transmitted light, is of a very deep 
reddish-violet colour. Diluted with more or less water, it gives 
at first a yellowish colour, and afterwards, both by transmitted 
and reflected light, a reddish-violet colour. It is to be observed 
that the reddish-violet colour does not become very evident for 
half, or sometimes one hour, and after some hours have elapsed, 
the tint is still stronger; it then becomes almost as deep as the 
colour produced by the alkalies. If the acetic acid is not pure 5 
if, for example, it contains sulphurous or sulphuric acid, which 
sometimes occur in acetic acid as usually prepared, their presence 
is easily detected by Brazil wood paper. Sulphurous acid destroys 
the action of acetic acid, or renders it extremely weak, according 
to the quantity which it contains, and sulphuric acid causes th 
acetic acid to give a yellowish colour, instead of the reddish 
violet. By this method, very small quantities of sulphuric acid 
may be discovered; acetic acid, for instance, which contains 
only 0-005 of sulphuric acid, gives a very evident yellowish 
colour. 
Citric acid, whether concentrated or diluted, gives a fine 
yellow colour, which is as durable as that occasioned by phos- 
phoric acid. 
