62 
POrULAR SCIENCE REVIEW. 
when boiled with madder, while cloth impregnated with iron 
hydrate acquires the purple colour so common in cotton prints. 
The composition of alizarin has now to be considered, for it 
was this that first suggested that it might be made artificially. 
Alizarin has been repeatedly analysed by different chemists, but 
with conflicting results. The variations in the results were no 
doubt due in part to the alizarin examined being of different 
degrees of purity, but partly to the fact that it is difficult to 
determine by unaided analysis the ratio of the hydrogen to 
the carbon in organic substances of complex constitution with 
sufficient accuracy to enable it to be expressed by numbers of 
atoms. Schunck gave alizarin one chemical formula, and 
Strecker gave it another inconsistent with this ; and now that 
the analogy of alizarin in properties to certain other bodies has 
led to its being again examined with the powerful aid that 
such analogy always furnishes, chemists have come to learn its 
true composition, and that both Schunck’s and Strecker’s 
formulae are incorrect. This brings us to the consideration of 
the way in which this valuable substance can be obtained in- 
dependently of the producing powers of the rubiaceae. 
After carrying out some investigations on a different subject 
altogether, Grraebe and Liebermann recognised the analogy 
just referred to of alizarin to the members of a class of bodies 
they had been studying, and were in consequence led to try 
the effect of heating alizarin with powdered zinc. This they 
found to be the production of a body already well known to 
chemists, called anthracene. 
Anthracene is a soft, white lamellated body without taste or 
smell, which can be obtained from coal or wood. Most persons 
are aware that when coal is heated in a distilling vessel, be- 
sides illuminating gas and other matters, a large quantity of 
the black, offensive-smelling, viscid liquid known as tar comes 
over. This tar is a mixture of many different substances, and 
these, by redistilling the tar, can be partly separated from each 
other. On account of the value of some of these substances 
the distillation of tar constitutes an important branch of in- 
dustry. The substances that first come over are mobile liquids, 
a mixture of which goes by the name of naphtha. Towards 
the end of the distillation the bodies that come over are semi- 
solid : among them is anthracene. 
The conversion of alizarin into anthracene at once threw 
light upon the chemical relations of the former body, and the 
chemists who had formed anthracene from alizarin next en- 
deavoured secundum artem to reverse this transformation, and 
get alizarin from anthracene. 
It was already known that by boiling anthracene with nitric 
acid it could be converted into a body containing oxygen, and 
