[42] The transparent varieties of sulphate of lime are distinguished by the name _Selenite_; and the fine massive varieties are called _Alabaster_. Gypsum forms very extensive beds in secondary countries, and is found in tertiary deposits; occasionally, in primitive rocks; it is also a product of volcanoes. The finest foreign specimens are found in the salt mines of Bex, in Switzerland; at Hall, in the Tyrol; in the sulphur-mines of Sicily; and in the gypsum formation near Ocana, in Spain. In England, the clay of Shotover Hill, near Oxford, yields the largest crystals.--See Dana’s _Mineralogy_, second edition, p. 241. CHAPTER VI. HEAT--SOLAR AND TERRESTRIAL. Solar and Terrestrial Heat--Position of the Earth in the Solar System--Heat and Light associated in the Sunbeam--Transparency of Bodies to Heat--Heating Powers of the Coloured Rays of the Spectrum--Undulatory Theory--Conducting Property of the Earth’s Crust--Convection--Radiation--Action of the Atmosphere on Heat Rays--Peculiar Heat Rays--Absorption and Radiation of Heat by dissimilar Bodies--Changes in the Constitution of Solar Beam--Differences between Transmitted and Reflected Solar Heat--Phenomena of Dew--Action of Solar Heat on the Ocean--Circulation of Heat by the Atmosphere and the Ocean--Heat of the Earth--Mean Temperature--Central Heat--Constant Radiation of Heat Rays from all Bodies--Thermography--Action of Heat on Molecular Arrangements--Sources of Terrestrial Heat--Latent Heat of Bodies--Animal Heat--Eremacausis--Spheroidal State Cold--Condensation--Freezing--Theories of Heat--Natural Phenomena--and Philosophical Conclusion. We receive heat from the sun, associated with light; and we have the power of developing this important principle by physical, mechanical, and chemical excitation, from every kind of matter. Our convictions are, that the calorific element, whether derived from a solar or a terrestrial source, presents no essential difference in its physical characters; but as there are some remarkable peculiarities in the phenomena, as they arise from either one or the other source, it will assist our comprehension of this great principle, if we consider it under the two heads. Untutored man finds health and gladness in the warmth and light of the sun; he rears a rugged altar, and bows his soul in prayer, to the principle of fire, which in his ignorance he regards as the giver and the supporter of life. The philosopher finds life and organization dependent upon the powers combined in the sunbeam; and, examining the phenomena of this wonderful band of forces, he is compelled to acknowledge that the flame upon the altar--on the Persian hills,--was indeed a dim shadow of the infinite wisdom which abides behind the veil.