THE ACTIONS OF FORCES ON ORGANIC MATTER. 27 



12. As elsewhere shown (First Principles, 103) Heat, or 

 a raised state of molecular vibration, enables incident forces 

 more easily to produce changes of molecular arrangement in 

 organic matter. But besides this, it conduces to certain vital 

 changes in so direct a way as to become their chief cause. 



The power of the organic colloids to imbibe water, and to 

 bring along with it into their substance the materials which 

 work transformations, would not be continuously operative 

 if the water imbibed were to remain. It is because it escapes, 

 and is replaced by more containing more materials, that the 

 succession of changes is maintained. Among the higher 



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animals and higher plants its escape is facilitated by evapor- 

 ation. And the rate of evaporation is, other things equal, 

 determined by heat. Though the current of sap in 



a tree is mainly caused by some action, probably osmotic, 

 that is at work in the roots ; yet the loss of water from 

 the surfaces of the leaves, and the consequent absorption 

 of more sap into the leaves by capillary attraction, must 

 largely aid the circulation. The drooping of a plant when 

 exposed to the sunshine while the earth round its roots is 

 dry, shows us how evaporation empties the sap-vessels ; and 

 the quickness with which a withered slip revives on being 

 placed in water, shows us the part which capillary action 

 plays. In so far then, as the evaporation from a plant's sur- 

 face helps to produce currents of sap through the plant, 

 we must regard the heat which produces this evaporation 

 as a part-cause of those re-distributions of matter which 

 these currents effect. In terrestrial animals, heat 



similarly aids the changes that are going on. The exha- 

 lation of vapour from the lungs and the surface of the skin, 

 forming the chief escape of the water that is swallowed, 

 conduces to the maintenance of those currents through the 

 tissues, without which the functions would cease. For 

 though the vascular system distributes nutritive fluids in 



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ramified channels through the body ; yet the absorption of 

 these fluids into tissues, partly depends on the escape of fluids 



