The autumnal equinox has arrived, with its equal day and night, as in the spring, but now the whole northern hemisphere is opposite the sun; hence, according to the laws already explained, we see the causes of the increased heat of the autumnal season. The winter solstice has come, with its long night and shortened day. The time during which radiation is going on is nearly twice that in which absorption takes place, and the earth is in her worst position for receiving heat, as that half which has the largest surface of water is towards the sun. These are the causes which lead to the variations of the seasons, and through these we learn why we are colder when near the sun than when at a considerably greater distance. 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.