An analysis of the spectrum shows us that there are some changes regularly taking place in the state of the solar beam, which cannot be referred to the mere alteration of position. It may be inferred, from facts by long-continued observations, that the three classes of phenomena--light, heat, and chemical power, distinguished by the term Actinism--which we detect in the sun’s rays, are constantly changing their relative proportions. In spring, the chemical agency prevails; in summer, the luminous principle is the most powerful; and in the autumn, the calorific forces are in a state of the greatest activity.[52] The importance of these variations, to the great economy of vegetable life, will be shown when we come to examine the phenomena connected with organisation.
A remarkable change takes place in the character of heat in being reflected from material substances. In nature we often see this fact curiously illustrated. Snow which lies near the trunks of trees or wooden poles melts much quicker than that which is at a distance from them, the sun shining equally on both--the liquefaction commencing on the side facing the sun, and gradually extending. We see, therefore, that the direct rays of solar heat produce less effect upon the snow than those which are radiated from coloured surfaces. By numerous experiments, it has been shown that these secondary radiations are more abundantly absorbed by snow or white bodies than the direct solar rays themselves. Here is one of the many very curious evidences, which science lays open to us, of the intimate connection between the most ethereal and the grosser forms of matter. Heat, by touching the earth, becomes more earth-like. The subtile principle which, like the spirit of superstition, has the power of passing, unfelt, through the crystal mass, is robbed of its might by embracing the things of earth; and although it still retains the evidences of its refined origin, its movements are shackled as by a clog of clay, and its wings are heavy with the dust of this rolling ball. It has, however, acquired new properties, which fit it for the requirements of creation, and by which its great tasks are facilitated. Matter and heat unite in a common bond, and, harmoniously pursuing the necessities of some universal law, the result is the extension of beautiful forms in every kingdom of nature.