Summary: A simple apparatus is described for the collection of carbon dioxide in baryta solution, and the subsequent filtration and washing of the barium carbonate produced with complete exclusion of atmospheric carbon dioxide. No special apparatus is required, the arrangement being set up from ordinary laboratory equipment; the manipulation of the apparatus is simple and much more convenient than the use of absorption tubes. The accuracy attainable is very satisfactory. The apparatus can be adapted for microanalysis. The effect of volatile acids other than carbonic has been tested experimentally; only hydrofluoric acid is liable to cause any interference, and a method of correcting for this, if necessary, is described.