From Pierre-Louis Dulong   5 août 1816

If I had delayed the reading of my paper for only one week, I would have lost the priority for several rather important items. A paper has been received from Berzelius that deals with the analysis of phosphates, of phosphoric acid, and phosphorous acid. We have agreed in very few respects on the topics we both considered. He makes no mention of phosphatic acid nor of hypophosphorous acid. Immediately following the reading of my paper, Gay-Lussac announced verbally to the Académie that there are likewise four sulphur acids, namely, sulphuric and sulphurous acid, which have been known for some time, and two new acids, which exist in the sulfuretted sulfites (thiosulfates). However, up to now he has not been able to isolate these acids. Your hypothesis as to the composition to the thiosulfates is applicable to the hyposulfites, but in order for it to acquire any credence it will be necessary at least to obtain by itself the phosphoretted hydrogen gas which is combined with phosphorous acid in hypophosphorous acid. However, my analyses show that the phosphoretted hydrogen gas which is produced under this condition ought to contain four times as much phosphorus as that which Davy has analysed. I know perfectly well that the gas which bursts into flame spontaneously contains more phosphorous than that Davy studied, but it is not at all probable that it contains four times as much. Furthermore, I do not claim that this would be impossible, and when I had determined the relation of this acid to the other compounds of phosphorous and oxygen I immediately suspected that it was not a binary acid. I plan to clear up this doubtful point by new experiments.