Two years ago the author submitted to the British Association a preliminary report on the subject, which has not been prosecuted, owing to the exhaustive investigations of Professor Thomsen of Copenhagen, on thermal values. In the paper referred to, the results are calculated on the assumption of hydriodic acid evolving 15,000 heat units for equivalent in aqueous solution. The above number is much too high, according to Thomsen's recent experiments, who gave 13,170 as the true number.