We have already noticed some remarkable differences between solar and terrestrial heat; but a class of observations by Delaroche[63] still requires our attention. Solar heat passes freely through colourless glass, whereas the radiations from a bright fire or a mass of incandescent metal are entirely obstructed by this medium. If we place a lamp or a ball of glowing hot metal before a metallic reflector, the focus of accumulated heat is soon discovered; but if a glass mirror be used, the light is reflected, but not the heat; whereas, with the solar rays, but little difference is detected, whether vitreous or metallic reflectors are employed. It is well known that glass lenses refract both the light and heat of the sun, and they are commonly known as burning-glasses: the heat accumulated at their focal point being of the highest intensity. If, instead of the solar beam, we employ, in our experiments, an intense heat produced by artificial means, the passage of it is obstructed, and the most delicate thermometers remain undisturbed in the focus of the lens. Glass exposed in front of a fire becomes warm, and by conduction the heat passes through it, and a secondary radiation takes place from the opposite side.[64] It has been found that glass is transcalescent, or _diathermic_, to some rays of terrestrial heat, and _adiathemic_, or opaque for heat, to others[65]--that the capability of permeating glass increases with the temperature of the ignited body--and that rays which have passed one screen traverse a second more readily. It would, however, appear that something more than a mere elevation of temperature is necessary to give terrestrial heat-radiations the power of passing through glass screens, or, in other words, to acquire the properties of solar heat. To give an example. The heat of the oxy-hydrogen flame is most intense, yet glass obstructs it, although it may be assisted by a parabolic reflector. If this flame is made to play upon a ball of lime, by which a most intense light is produced, the heat, which has not been actually increased, acquires the power of being refracted by a glass lens, and combustible bodies may be ignited in its focus. It certainly appears from these results, that the undulatory hypothesis holds true, so far as the motion of the calorific power is concerned. At a certain rate the vibrations are thrown back or stopped by the opposing body, while in a state of higher excitation, moving with increased rapidity, they permeate the screen.[66] This does not, indeed, interfere with the refined theory of Prévost,[67] which supposes a mutual and equal interchange of caloric between all bodies.