Paper, 'An investigation of a case of gradual chemical change' by W H Pendlebury and Margaret Seward The authors write: 'When substances which act upon each other are brought together under suitable conditions, a change takes place which consists in the disappearance of the original substances and the production in their place of an equal weight of other substances. The change proceeds till the whole of that reacting substance which was present in the smallest relative quantity has disappeared. This process may take a long time, as in the case which forms the subject of the present investigation, or the limit may be reached so rapidly that the change seems instantaneous. This difference, however, is one of degree and not of kind. In the present case the masses of the substances mixed together were so large relatively to the masses undergoing change during the time over which the observations extended, that the masses of reacting substances were practically constant. Thus it happens that each set of observations was of a change proceeding constant velocity.' Annotations in pencil and ink. Includes one photograph and diagram of experimental apparatus and a graph of experimental results. Subject: Chemistry Received 27 November 1888. Read 13 December 1888. Communicated by [Augustus] Vernon Harcourt. A version of this paper was published in volume 45 of the Proceedings of the Royal Society as 'An investigation of a case of gradual chemical change: the interaction of hydrogen chloride and chlorate in presence of potassium iodide'. 1888