The experiments of Professor Crum Brown and the author on the “Thetines” and their derivatives have clearly shown that very striking analogies exist between certain compounds of nitrogen and sulphur. Thus sulphide of methyl closely resembles trimethylamine (and ammonia), in many of its reactions, and in the products which it gives rise to. Like trimethylamine it combines with a molecule of bromacetic acid, and the resulting product, which was named hydrobromate of dimethyl-thetine, behaves in certain respects like the compound of bromacetic acid and trimethylamine (hydrobromate of betaine).

Analogous phosphorus compounds have been obtained: in the ethyl series by Hofmann, in the methyl series by A. H. Meyer. The latter compound is simply the betaine salt in which the nitrogen is replaced by phosphorus, and may be called a salt either of phosphorus-betaine or of trimethyl-phosphorus-betaine.