The pyromeconic acid employed in the following experiments was obtained by distilling impure meconic acid at a temperature of about 500° or 600° Fahr., when there is obtained a highly crystalline sublimate of a dark colour and empyreumatic odour. Its purification was effected by pressing the crystals so procured between folds of filtering paper, and finally subliming them at a moderate heat in cylindrical glass vessels provided with paper diaphragms.