Light is not, however, alone sufficient to perfect the plant: another agent is required to aid in the production of flowers and fruits, and this power is proved to be heat--and heat, perhaps, in some peculiar condition. Having reached that point of development when the reproductive functions are, by another change in the chemical operations going on within the vegetable structure, to be called forth, it has been found that the heat rays become in a remarkable manner effective. It has also been observed that plants bend from the red, or calorific rays, instead of towards them, as they are found to do to every other ray of the spectrum. From this we may argue that the influence of these rays is to check the vegetation, and thus to ensure the perfection of the reproductive processes. It has already been stated that we have the means of separating, to a considerable extent, the three principles which we discover in the sunbeam, from each other, by the use of absorbent media. By a peculiar yellow glass we cut off the chemical principle of the sunbeam, and admit the passage of the _luminous rays_ only--LIGHT. By a cobalt blue glass we obstruct the _light_, but allow the chemical agent to pass through freely, without, indeed, any loss--ACTINISM. By a glass coloured deep blood-red by oxide of gold we obstruct the chemical principle and much of the light, but such a medium is perfectly transparent to HEAT. Therefore, this gives us the means of experimenting with either of these principles, and of examining the parts which they respectively play in the work of organization. Some seeds being placed in the soil, in every respect in their natural conditions, duly supplied with moisture, and a uniform and proper temperature maintained, we place above the soil the three media above named, and allow one portion to be exposed to all the ordinary influences of the solar rays. The result will be, that the seeds under the blue glass will germinate long before those which are exposed to the combined influences of the sunshine: a few of the seeds will struggle into day under the red glass, but the process of germination is entirely checked under the yellow glass. Here we see that the chemical radiations have quickened the chemical changes, and accelerated the process, under the red glass, through which rays having some peculiar chemical action pass; the germinating process, though checked, is not entirely stopped. Whereas, it would appear that under the influences of light which has been deprived of chemical power, this conversion of the starch into gum and sugar, which appears to be necessary, is entirely prevented.