Gifford hears in Stephen's "a chemistry of stars" an allusion to certain alchemical ideas, alchemy having been popular with the turn-of-the-century Theosophists. In particular, he cites a distinction in Madame Blavatsky’s The Key to Theosophy (1893) between lesser alchemists who seek only to transmute baser metals into gold and the better kind who, “spurning the gold of the earth,” seek to transmute the material aspects of human existence into spiritual realities.

Alchemical ideas do occasionally surface in Stephen's thoughts. But since Blavatsky's book does not mention stars or other heavenly bodies, Gifford's attribution seems conjectural. 

JH 2011

Helena Petrovna Blavatsky. Source: www.havelshouseofhistory.com.