The application of the de Saladin-Le Chatelier differential thermal analysismethod to the identification of soil clays has given rise to a certainnumber of difficulties. In fact, it is possible for these clays to bealtered either in situ by the action of atmospheric and biochemical agentsor in the laboratory during their extraction. Mr Henin and I have beeninvestigating what influence chemical agents can exert on the thermaldiagrams of different types of clays. The treatments have been effected bynormal solutions of cations in the form of chlorides, carbonates andnitrates, or by N/10 acid or basic solutions.