2056. In particular situations heat may be obtained from anomalous sources, as in Iceland, Toplitz, and Matlock, from hot springs; and perhaps in some cases, especially in coal districts, from a basement composed of certain compounds of sulphur and iron, &c. Dr. Anderson (Treatise on the Patent Hothouse) proposed to preserve the superfluous heat generated by the sun in clear days, and to retain it in reservoirs placed under, above, or at one side of the house, re-admitting it as wanted to keep up the temperature; but the plan, though ingenious and philosophical, required too much nicety of execution, and the clear days in this country are too few to admit of adopting it as a substitute for heating by ignition. Heat must not only be produced in hothouses, but its waste avoided, by forming as large a portion of the cover as possible of materials through which it escapes with difficulty, as far as this is consistent with other objects. Hence, in certain classes of houses, the side to the north is formed of opaque and non-conducting materials,
