 Although the law of constant proportions has not survived the discovery of Berthollides and more careful analyses of solutions showed that chemical combination or affinity is not confined to compounds, it gave chemists a principled way of studying how elements combine to form compounds through the 19th century. This account of Berthollides also illustrates the interplay between macroscopic and microscopic theory which is a regular feature of modern chemistry, and which we turn to in the next section.