Each one of you will doubtless be aware that the title of this lecture is taken from Goethe's Faust. You all know that in this poem we are shown how Faust, the representative of the highest human effort, enters into a pact with the evil powers, who on their side are represented in the poem by Mephistopheles, the emissary of hell. You will know, too, that 'Faust is to strike a bargain with Mephistopheles, the deed of which must be signed with his own blood. Faust, in the first instance, looks upon it as a jest. Mephistopheles, however, at this juncture utters the sentence which Goethe without doubt intended should be taken seriously: "Blood is a very special fluid."