ON COLOURING MATTERS. 
141 
From some analyses which I have made of this acid, I infer that its com¬ 
position is expressed by the formula 0 „ and that the formula for 
pj'ro-aJizarie acid is C 14 H 3 O 5 , in which case alizaric acid contains one equi¬ 
valent of hydrogen less and three equivalents of oxygen more than benzoic 
acid, which is C,^ O 4 . 
This is the substance which I called alizarin in the paper which 
I read on this subject last year. I have since discovered that it is not iden¬ 
tical with Robiquet's alizarin, and have therefore given it a new name. 
Knbiacin is most easily obtained in a stale of purity from rubiacic acid. 
Rubiacin andnibiacic acid are mutually convertible, as they only diJIer from 
one another by a certain number of atoms of oxygen, llubiacin i« converted 
into rubiacic acid by the action of tlie pcrsalts of iron, and rubiacic acid may 
be reconverted into rubiacin by reducing agents, such as sulphuretted hydro¬ 
gen. In order then to obtain pure rulnacin, it U best to take a aolution of 
purr rubiocate of potash, to which a slight excess of caustic potii.«ii has been 
added, and to pass sulphuretted hydrogen gas through the solution for some 
time. Chloride of barium is then added, which produces a dark purple pre¬ 
cipitate, consisting of rubiacin in combination with baryta. ThU is separated 
by filtration, and w’ashed with cold svatcr until the sulphuret of barium and 
hyposulphite of baryta are removed, and Ibeji dec«mip»tscd with muriatic 
acid, which leaves the rubiacin behind. After being wjL«hed with water, it 
is dissolved in boiling alcohol, from which it crystullizes ou the solution 
cooling. It is obtained by these means in beautiful yellow crystalline plates 
and nw'dles, having a strong lustre. The colour is n little darker than that 
ot neutral chromate of potash, and incliiies more to green than to red. If 
there is a tinge of red in it, it is impure, and must be recrystallizcd. It has 
the following properties : — 
It ia a little soluble in boiluig water, to which it communicates a reddish- 
yellow tinge. The solution deposits it on cooling in shining yellow scales. 
The alcoholic solution has a pure yellow colour, without a tinge of red. It 
“ precipitated from its alcoholic solution by water in yellow flocks. When 
heated on platinum foil it melts, and burns with a smoky flame, leaving no 
fi«d residue. When heatetl in a tube closed at one end, it melts and gives 
yellow fumes, which condense in the colder {)arts of the tube in yellow cry- 
If it be heated gradually at a moderate Ujuiperaturo, it may be almost 
volatilized, leaving only a fllin of cJiarconl. Concentrated sulphuric 
^■'d dissolves it with a yellow colour, and water precipitates it again in yellow 
nocKg. Tjip gyiytjfjn sulphuric acid may Ik* heated to the boiling-point 
p becoming in the least degree black, and watt:r still prtTipitates it. 
wicentrated nitric acid slowly decomposes it on boiling with an evolution 
0 aitrous acid. Dilute nitric aci<l dissolves it with a yellow colour ou boiling, 
“ on the solution cooling, it crystallizes out again in shining needles. It 
^^ars therefore that nitric acid docs not convert it into rubiacic acid, 
'hag acetic achl liissolvcs it and deposits it again ou cooling in yellow 
It is afFecte<l by chlorine, Itdissolves in a concentrated solution 
or pernitrate of iron, fonuing 0 dark brownish-red solution, 
ich oti the aildition of acid becomes yellow, and deposits yellow flocks. 
^ "“cks are no longer uncJiangcd rubiacin, but rubiacic acid. It is a 
rcDiarkable eircuinstancc, that though rubiacic acid differs from rubiacin 
^®^taioiug a few atoms more oxygen, it is not formed from the 
ntnA action of nitric acid cither dilute or concentrated, but is easily 
iom 1 ?*^ pcrsalts of iron, which seems to indicate that 
besides the influence of oxygen is brought into play during the 
process. Rubiacin is soluble in carbonate of potash with a blood-red colour; 
