 Chemists’ attention was largely directed towards the investigation of compounds in the first half of the 19th century, initially with a view to broadening the evidential basis which Proust had provided. For a time, the law of constant proportions seemed a satisfactory criterion of the occurrence of chemical combination. But towards the end of the 19th century, chemists turned their attention to solutions. Their investigation of solutions drew on the new science of thermodynamics, which said that changes of state undergone by substances when they are brought into contact were subjected to its laws governing energy and entropy.