The analogy was that of the catalyst. When the two gases previously mentioned are mixed in the presence of a filament of platinum, they form sulphurous acid. This combination takes place only if the platinum is present; nevertheless the newly formed acid contains no trace of platinum, and the platinum itself is apparently unaffected; has remained inert, neutral, and unchanged. The mind of the poet is the shred of platinum. It may partly or exclusively operate upon the experience of the man himself; but, the more perfect the artist, the more completely separate in him will be the man who suffers and the mind which creates; the more perfectly will the mind digest and transmute the passions which are its material. The experience, you will notice, the elements which enter the presence of the transforming catalyst, are of two kinds: emotions and feelings. The effect of a work of art upon the person who enjoys it is an experience different in kind from any experience not of art. It may be formed out of one emotion, or may be a combination of several; and various feelings, inhering for the writer in particular words or phrases or images, may be added to compose the final result. Or great poetry may be made without the direct use of any emotion whatever: composed out of feelings solely. Canto XV of the _Inferno_ (Brunetto Latini) is a working up of the emotion evident in the situation; but the effect, though single as that of any work of art, is obtained by considerable complexity of detail. The last quatrain gives an image, a feeling attaching to an image, which “came,” which did not develop simply out of what precedes, but which was probably in suspension in the poet’s mind until the proper combination arrived for it to add itself to. The poet’s mind is in fact a receptacle for seizing and storing up numberless feelings, phrases, images, which remain there until all the particles which can unite to form a new compound are present together.