Comparative Physiology. 339 



ceptible on their being magnified. It appears to act principally 

 by drying up the fixed water which all seeds contain, and which 

 seems necessary to keep up the capability of exhibiting vital 

 powers in seeds ; all seeds, when bruised in a fresh state, exhibit 

 generally a moist appearance in the albumen, which assumes a 

 dry floury appearance in seeds hurt by dry heat. The vitality 

 probably depends on the capability of exhibiting chemical action, 

 or it may be called electrical following from chemical ; and the 

 fixed water, or moist state of the seed, probably is needful in 

 keeping the tissue in a state fit for exhibiting vital chemical 

 properties. When kept from the drying power of air they keep 

 long. Some seeds will stand a great deal of moist heat without 

 being killed, as I noticed before in my essay on the Theory of 

 Horticulture, in adverting to the power of hastening germination 

 in seeds by boiling water : the vesicles of starch are always rup- 

 tured in ordinary germination. Heat appears the principal agent 

 in evaporation, the red or heating end of the spectrum, which 

 is always in the positive or plus state of electricity, has most 

 momentum, and will pass most easily through a refracting medium, 

 as glass ; and the concenti'ated rays in curvilinear houses, which 

 are found to destroy plants, probably act more from the evapora- 

 tion, &c., by heat than from the chemical power of light, to which 

 it has been ascribed ; the blue chemical rays do not pass so easily, 

 and are not so likely to be in excess. Heat increases the intensity 

 of light : by throwing heat on the metal of a jet of light, it has 

 been found to cause white light, while, by throwing cold on 

 the metal, it was found to cause a faint blue light. Professor 

 Lardner says it is still uncertain whether heat and light be the 

 same principle manifesting itself in different ways, or distinct 

 physical agents having the same nature. Glass stops more of 

 the heat of the fire than of the sun light, when held between it 

 and an obj ect, and has been thought to distinguish ; but this 

 arises, he says, from the heat being so much greater in propor- 

 tion in the ray from the fire than in that from the sun. 



Cold diminishes vital activity, the great source of health and 

 vigour ; and diminished vital activity is very apt to end in dis- 

 ease, especially if applied in a previous active state of the vital 

 powers. If the tissue is young and succulent, and full of fluid 

 in the plant, it expands the juices by freezing, and bursts the 

 vessels, causing death of the part affected, and injuring the 

 whole system of the plant before new shoots are evolved. It 

 has been said that the danger is greatest from the excitement 

 produced by next day's heat, and that the air in the air vessels, 

 condensed by cold, and occupying thus less bulk, gives room 

 for the expansion of the cells containing frozen juice ; and that 

 it is not till sunrise in the ensuing day, by again expanding the 

 air in the air vessels, destroys this balance, that danger takes 



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