64 
POPULAR SCIENCE REVIEW. 
Both the natural and artificial bodies crystallize in needles, 
which are usually curved, especially when small. 
When dissolved in caustic alkali, they both form violet solu- 
tions of the same tint. 
When applied to mordanted fabrics, they produce exactly 
the same colours, bearing the treatment with soap equally. 
They also possess the same tinctorial value. 
When dissolved in alcohol, they produce with cupric acetate 
a purple solution of precisely the same shade of colour. 
When examined with the spectroscope, their potassic solu- 
tions produce the same absorption bands. 
Lastly, the ordinary precipitated artificial alizarin, yields 
phthalic acid when decomposed with nitric acid, just as alizarin 
from madder has long been known to do. 
There being no other well defined reaction of alizarin, we 
are, therefore, judging from the above, bound to consider arti- 
ficial and natural alizarin as identical. 
In addition to alizarin another colour, called purpurin, is 
obtainable from madder. The existence of purpurin has 
indeed been denied by Schunck, but, after all, this denial has 
not so much practical importance, as he only contends that the 
substance obtained and called purpurin is nothing but a com- 
bination of alizarin with another subst&nce^verantine, and not 
an entirely distinct substance. Now the body called purpurin 
has been considered to be a material ingredient of the finer 
colours obtained from madder ; and if this were the case, arti- 
ficial alizarin would prove to be only an imperfect substitute 
for madder. Schunck, however, also denies that anything but 
alizarin enters into the formation of madder-colours. Then it 
has been pointed out by Stokes that the spectrum of purpurin 
is very different from that of alizarin, so that it becomes easy 
to detect minute quantities of the former in this way ; yet on 
applying this test to fabrics dyed with madder no purpurin can 
be detected. 
According to Perkin there can be no doubt that the more 
brilliant the colours dyed with madder, the purer is the alizarin 
in combination with the mordants. This objection therefore 
to the substitution of artificial alizarin for madder may be 
safely regarded as of no value whatever. 
The difficulty that remains to be solved is as to how anthra- 
cene may be got in sufficiently large quantity to make artificial 
alizarin an economical substitute for madder. Now that there 
is a special demand for this substance, we may fairly anticipate 
that manufacturers will, before very long, succeed in producing 
it in much larger quantities than at present by the distillation 
of coal. It is already known that at a sufficiently high tem- 
perature various other hydrocarbons, alone or mixed together, 
