* Viewless floods of beat. l. 462. The fluid matter of heat, or Calorique, in which all bodies are immersed, is as necessary to vegetable as to animal existence. It is not yet determin∣able whether heat and light be different materials, or modifications of the same materials, as they have some properties in common. They appear to be both of them equally ne∣cessary to vegetable health, since without light green vegetables become first yellow, that is, they lose the blue colour, which contributed to produce the green; and afterwards they also lose the yellow and become white; as is seen in cellery blanched or etiolated for the table by excluding the light from it.

The upper surface of leaves, which I suppose to be their organ of respiration, seems to require light as well as air; since plants which grow in windows on the inside of houses are equally sollicitous to turn the upper side of their leaves to the light. Vegetables at the same time exsude or perspire a great quantity from their leaves, as animals do from their lungs; this perspirable matter as it rises from their fine vessels, (perhaps much finer than the pores of animal skins,) is divided into inconcievable tenuity; and when acted upon by the Sun's light appears to be decomposed; the hydrogene becomes a part of the vegetable, composing oils or resins; and the Oxygene combined with light or calo∣rique ascends, producing the pure part of the atmosphere or vital air. Hence during the light of the day vegetables give up more pure air than their respiration injures; but not so in the night, even though equally exposed to warmth. This single fact would seem to shew, that light is essentially different from heat; and it is perhaps by its combination with bodies, that their combined or latent heat is set at liberty, and becomes sensible. See additional note, XXXIV.

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