When I was invited by Christ's College to deliver this year the Liversidge Lecture, it was indicated to me kindly but firmly that I was to talk about heavy hydrogen. I appreciate most deeply the honour you have done me by this invitation, especially by coupling my name (as it were) with this distinguished American newcomer to physical and chemical science. I cannot claim myself to be much more than an interested spectator of the rapid progress which has already been made in the study of this new substance, and the facts that I shall comment on (and the many others which time will force me to neglect) have been established by the collaboration of many workers—some of them our colleagues here, who would be much better qualified to speak at first hand about heavy hydrogen than I. Professor Liversidge laid it down in his deed of gift that his lecturer shall not deal with generalities or give a mere review of his subject, or give an instructional lecture suitable for undergraduates, but shall primarily try to encourage research and to stimulate himself and his audience to think, and to acquire new knowledge. In trying to fulfil his wish I shall not therefore catalogue all the new interesting properties of heavy hydrogen; this will mean, I fear, that I can make little or no further reference to its extremely exciting properties as an atomic projectile in disintegrating lithium and other nuclei, or to the equally exciting prospect that heavy water may have entirely distinct biological properties from ordinary water, being even lethal to many organisms, or to the wonderful possibilities of the new complex organic chemistry.