It is evident from this table that sand is very retentive of heat, while clays and humus are very cold. Green vegetation lessens the absorbing power of the soil, and radiation of heat is more rapid, evaporation occurring constantly from the herbage. The influence of trees on the temperature of the soil is considered on page 228. Damp soils are colder than dry soils because of the evaporation going on. Buchan finds as the result of drainage of the soil, that (1) the mean temperature of arable land is raised 0·8° Fahr.; (2) cold is propagated more quickly through undrained land; (3) drained land loses less heat by evaporation; (4) the temperature of drained land is more equable, and (5) in summer is often 1·5° to 3° above that of undrained land. DISEASES ARISING FROM THE SOIL.—The soil may be a cause of disease: (_a_) indirectly and (_b_) directly. _Indirectly_ a damp soil may cause disease by acting as a means of lowering the vitality of man and diminishing his resistance to disease. It is in this way that it has been credited with causing such diseases as neuralgia, catarrhs, and rheumatism. It is one of the elements in producing a climate unfavourable to health. As to rheumatism, see page 225. _Directly_ the soil may transmit the actual contagia (micro-organisms) of disease either by means of the subsoil water or its air. In the former case the disease-causing material gains access to the drinking water of wells, springs, or rivers; in the latter case it may be borne to the surface of the soil by currents of the ground-air or by insects, and then inhaled as dust, or gain access to food. Certain disease-producing micro-organisms have been proved to be capable of living for some time in the soil. The chief of these found in the soil are the bacilli of tetanus (lockjaw), of anthrax, of malignant oedema, and of enteric (typhoid) fever. There are reasons for thinking also that the micro-organisms causing diphtheria, rheumatic fever, and epidemic diarrhœa, and possibly some other diseases, may occasionally live in the soil. In some diseases as enteric fever, cholera, dysentery and anthrax, the contamination of the soil can be shown to be derived from a patient suffering from the same diseases. In others, and particularly in tetanus, the same chain of evidence is obtainable. (1) The conditions favourable to the production of =malarial diseases= have been generally considered to be the presence of a certain proportion of dead organic matter, the exposure of the soil to alternations of heat and moisture, with a limited access of air, and a temperature of at least 65°F. Though most common in marshy districts, and in recent alluvial soils, malaria may develop in connection with any geological formation. That it may be removed by drainage of the subsoil, is well known. The true nature of the connection between soil and malaria is stated on page 220.