The picture which we receive to-day, unless we adopt some method of securing its permanency, fades away before the morrow, and we try to restore it in vain. With some of our chemical preparations this is very remarkably shown, but by none in so striking a manner as by paper prepared with the iodide of platinum, which, being impressed with an image by heliographic power, which is represented by dark brown tints, restores itself in the dark, in a few minutes, to its former state of a yellow colour, and recovers its sensibility to sunshine.[122] The inference we alone can draw from all the evidences which the study of actino-chemistry affords, is, that the hours of darkness are as necessary to the inorganic creation as we know night and sleep to be to the organic kingdom. But we must not forget that there does exist in the solar rays a balance of forces which materially modifies the amount of disturbing influence exerted by them on matter. Not only do we find that the chemical action is not extended over the whole length of the prismatic spectrum, but we discover that over spaces, which correspond with the maximum points of light and heat, a protective action is exerted. That is, that highly sensitive photographic agents, which blacken rapidly under exposure to diffused daylight, are entirely protected from change in full sunshine, if at the same time as a strong light is thrown upon them by reflection, the yellow and extra red rays are brought to bear upon their surface. Not only so, but by employing media which will cut off all the chemical rays of the spectrum, admitting freely at the same time the luminous and calorific rays, we find that a protected band, the length of the spectrum, remains white, whilst every other portion has blackened.[123]