On that occasion he climbed from the pass up the Klein Matterhorn (3,883 m), while in 1792 he spent three days making observations on the same pass without descending to Zermatt and then visited the Theodulhorn (3,472 m). All of Saussure's observations and experiments from seven Alpine journeys were summed up and published in four quarto volumes, under the general title of "Voyages dans les Alpes (1779 – 1796)" (There was an octavo issue in eight volumes, issued from 1780 to 1796). The non-scientific portions of the work were first published in 1834, and often since, as "Partie pittoresque des ouvrages de M. de Saussure". The Alps were the focus of Saussure's investigations. He saw them as the grand key to the true theory of the earth, and they gave him the opportunity to study geology in a manner never previously attempted. Saussure closely examined the inclination of the strata, the nature of the rocks, the fossils and the minerals. Saussure had a thorough knowledge of the chemistry of the day and applied it to the study of minerals, water and air. His geological observations made him a firm believer in the Neptunian theory: He regarded all rocks and minerals as deposited from aqueous solution or suspension, and attached much importance to the study of meteorological conditions. His work with rocks, erosion, and fossils also led him to believe that the earth was much older than generally thought and formed part of the basis of Darwin's Theory of Evolution. Saussure carried barometers and boiling-point thermometers to the summits of the highest mountains, and estimated the relative humidity of the atmosphere at different heights, its temperature, the strength of solar radiation, the composition of air and its transparency. Then, he investigated the temperature of the earth at all depths to which he could drive his thermometer staves, and the course, conditions and temperature of streams, rivers, glaciers and lakes, even of the sea. Saussure adapted the thermometer to many purposes: for ascertaining the temperature of the air he used one with a fine bulb hung in the shade or whirled by a string, the latter form being converted into an evaporimeter by inserting its bulb into a piece of wet sponge and making it revolve in a circle of known radius, at a known rate; for experiments on the earth and in deep water he employed large thermometers wrapped in non-conducting coatings so as to render them extremely sluggish, and capable of long retaining the temperature once they had attained it.