M. Liebig has pointed out an important chemical resemblance between this Taurine and the vegetable alkaloids. Of these he has singled out Caffeine, the peculiar principle of Tea and Coffee, as most analogous to the biliary product. From the analysis of these two substances he has deduced the following formulæ.
Taurine = C4H7NO10.
Caffeine = C8HN2O2.
Then he shows that an atom of Caffeine, if added to 9 of water and 9 of oxygen, will form two atoms of Taurine. (Organic Chemistry, 1842, p. 180.)
It should be remarked that a different formula for Taurine has been arrived at by M. Lehmann and others, who assert that it contains Sulphur. Choleic acid, and other constituents of this complicated fluid, certainly contain sulphur. But even if Sulphur be combined with Taurine, it is not clear that it enters with it into the blood. And the above formula for Taurine has been confirmed by the analysis of M. Löwig.[34] It seems to be sufficiently correct for our purpose.
With the supposed function of Caffeine as a creator of bile, however probable or interesting, we are not now immediately concerned. We are engaged in inquiring whether there is naturally in the blood any substance which resembles the tonic alkaloids. Liebig has shown that this element of the bile is intended for re-absorption into the circulation. And without theorizing at all on the subject, it may readily be demonstrated that, with the existence in the blood of such a material as Taurine, we cannot say that we have not in that fluid an analogue to substances like Quina. Taurine is soluble in water, and crystallizable; is chemically analogous to the tonic bitter principles; and contains the same four elements as Quina. If necessary, this similarity could be exchanged for identity, and Quina could be formed into Taurine in the blood.
If there be one change in the blood to which, more than to any other, the substances which enter it are liable, it is certainly the process of oxidation. Free oxygen is continually entering the blood through the air-cells of the lungs.
The formula for Quina is C20H12NO2. Adding to this 45 atoms of oxygen, we could make out of it one atom of Taurine, together with 16 of Carbonic acid, and 3 of water.
C20H12NO2 + 45 O = C4H7NO10 + 16 CO2 + 5 HO.
Thus it is conceivable that this and similar alkaloids may at the same time supply an important material to the blood, and serve as fuel to support the animal heat, by combining with oxygen and giving off Carbonic acid. I would not, of course, affirm that this actually does occur; but if it were to take place, we might gain, in this oxidation of Quina, some clue to the production of that quickening of the pulse and increase of the general temperature, which are affirmed by some to follow the ingestion of the alkaloid, even in small doses. But it is now sufficient for us if we have ascertained that there is naturally in the system, and in the blood, a substance which resembles Quina. We may be allowed to infer that the presence of the latter substance in that fluid would not be unnatural to it.
Having dismissed the second of the minor propositions which relate to Tonics as Restorative medicines, we pass on to the third. Are Quina and the vegetable bitters necessarily excreted from the blood?