If we introduce six hydrogen atoms into pyridine, we convert it into piperidine.  Ladenburg succeeded in so hydrogenizing pyridine by acting upon an alcoholic solution with sodium, and from the base which was formed he obtained a platinochloride which agreed with the similar double salt of piperidine.  He has also prepared it from trimethyline cyanide by the action of sodium.  Pentamethylinediamine is the principal intermediary product, and this gives piperidine when distilled with superheated steam.  He has proved that the alkaloid so obtained is identical with that prepared from piperine.  Another curious point which Ladenburg has lately proved is that cadaverine (one of the products of flesh decomposition) is identical with pentamethylinediamine, and that its imine is the same as piperidine.  The synthesis of coniine by Ladenburg is one of the most notable achievements of modern chemistry.  He at first supposed that this alkaloid was piperidine in which two hydrogen atoms were replaced by the isopropyl radical (C_{3}H_{7}), its formula being taken as C_{5}H_{9}(C_{3}H_{7})NH.  But he has since changed his view, as will be seen from what follows.  In its synthesis 1,000 grammes of picoline were first converted into alphapicoline, 380 grammes being obtained.  This was heated with paraldehyde, whereby it was converted into allylpyridine (48 grammes), and this by reduction with sodium yielded alpha-propylpyridine, a body in almost every respect identical with coniine.  The more important difference was its optical inactivity, but he succeeded in splitting up a solution of the acid tartrate of the base by means of Penicillium glaucum.  Crystals separated which had a dextro-rotatory power of [a]_{D} = 31 deg. 87’ as compared with the [a]_{D} = 13 deg. 79’ of natural coniine.  This brief account conveys but a faint idea of the difficulties which were encountered in these researches.  Optical methods of examination have proved of great value, and are destined to play an important part in such work.

Among the most complex alkaloids are those of the quinine group.  As yet chemists have got no further with these than the oxidation products; but the study has afforded us several new antipyretics and many interesting facts.  It has been found, for example, that artificial quinine-like bodies, which fluoresce and give the green color with chlorine water and ammonia, have antipyretic properties like quinine, but their secondary effects are so pernicious as to prevent their use.  If, however, such bodies are hydrogenized or methylated they lose their fluorescing property, do not give the green color, and their secondary effects are removed.  Knowledge of these facts led to the discovery of thalline.  It is prepared from paraquinanisol, one of the objectionable bodies, by reduction with tin and hydrochloric acid.  The following formulae show the constitutional relationship of these compounds: