rays of the spectrum, whether luminous or thermic, which occupy its red, yellow, and green regions; while, on the other hand, it receives no such assistance from the purely thermic rays beyond the spectrum acting under similar circumstances and in an equal state of condensation. The conclusions drawn from these experiments are confirmed by that which follows: a photographic picture formed on paper prepared with a mixture of the solutions of ammonia-citrate of iron and ferro-sesquicyanite of potash in equal parts, then thrown into water and afterwards dried, will be blue and negative, that is to say, the lights and shadows will be the reverse of what they are in nature. If in this state the paper be washed with a solution of proto-nitrate of mercury, the picture will be discharged; but if it be well washed and dried, and a hot smoothing-iron passed over it, the picture instantly reappears, not blue, but brown; if kept some weeks in this state in perfect darkness between the leaves of a portfolio, it fades, and almost entirely vanishes, but a fresh application of heat restores it to its full original intensity. This curious change is not the effect of light, at least not of light alone. A certain temperature must be attained, and that suffices in total darkness; yet, on exposing to a very concentrated spectrum a slip of the paper used in the last experiment, after the uniform blue colour has been discharged and a white ground left, this whiteness is changed to brown over the whole region of the red and orange rays, _but not beyond_ the luminous spectrum. Sir John thence concludes:—1st. That it is the heat of these rays, not their light, which operates the change; 2ndly. That this heat possesses a peculiar chemical quality which is not possessed by the purely calorific rays outside of the visible spectrum, though far more intense; and, 3rdly. That the heat radiated from obscurely hot iron abounds especially in rays analogous to those of the region of the spectrum above indicated.