“…The exact mode of action of carbon dioxide in stimulating respiration has been the subject of considerable speculation and controversy. It was considered (Haldane, 1922;Banus, Corman, Perlo & Popkin, 1944) that increase in arterial C02 tension led to an increase in the concentration of arterial carbonic acid, and that this affected respiration by altering pH. This hypothesis was challenged by Nielsen (1936) on the grounds that the respiratory response to a given arterial pH change induced by ammonium chloride was considerably smaller than the response to a smaller pH change produced by carbon dioxide.…”