The light and heat poured forth from the sun are, as stated, in the form of radiated energy. They penetrate the attenuated vapors as far as vision extends, and doubtless farther, but they cannot reach the boundaries of space, for even the mind of man cannot reach those limits. Aqueous vapor absorbs heat; we know this without any demonstration, for the radiated heat of the earth is arrested by a veil of clouds, so that on cloudy nights frost will not form. So also the sun shining into water will raise its temperature, as in a glass globe, and such absorption of heat by aqueous vapors or water would be much more manifest were not a large part employed in loosening the tension of the constituent molecules, since, when thus employed, it is not manifest as sensible heat. Professor Tyndall, in "The Forms of Water," states that "The quantity of heat which would raise the temperature of a pound of water one degree would raise the temperature of a pound of iron ten degrees." Professor Stewart, in "The Conservation of Energy," says, "That peculiar motion which is imparted by heat when absorbed into a body is, therefore, one variety of molecular energy.... Part of the energy of absorbed heat is spent in pulling asunder the molecules of the body under the attractive force which binds them together, and thus a store of energy of position is laid up, which disappears again after the body is cooled. "Heat will only be changed into work while it passes from a body of high temperature to one of low.... At very high temperatures it is possible that most compounds are decomposed, and the temperature at which this takes place, for any compound, has been termed its temperature of disassociation. Heat energy is changed into electrical separation when tourmalines and certain other crystals are heated." It may be added that it is also changed into electrical energy by the operation of all electrical machines, as molecular motions are all mutually interconvertible, and heat itself is only a mode of such motion. Of radiant energy, the same writer says, "This form of energy [radiant heat] is converted into absorbed heat whenever it falls upon an opaque substance ... and heats it. It is a curious question to ask what becomes of the radiant light from the sun that is not absorbed either by the planets of our system or by any of the stars. We can only reply to such a question that, as far as we can judge from our present knowledge, the radiant energy that is not absorbed must be conceived to be traversing space at the rate of one hundred and eighty-eight thousand miles a second."