PRESENT FUNDAMENTAL CONCEPTIONS OF PHYSICS. 517 



converted into mechanical work, as in tbe case of tbe steam engine, and 

 as demonstrated by calculations, to more advantage than in tbe latter, 

 since tbe animal body provides in addition for its involuntary motions. 

 According to tbe theory of tbe conservation of energy, a man, when 

 working, must, under otherwise unchanged conditions, expend more heat 

 than when resting, consequently a more considerable oxidation or a 

 greater consumption of oxygen must ensue. 



Through the continuous process of oxidation tbe products thereof, 

 namely, carbonic acid and ammonia, would accumulate in a threatening 

 manner if plants did not put them to use in their construction, partly 

 directly and partly absorbed in water. Tbe carbonic acid consumed by 

 plants is decomposed under the influence of the sun's light, the carbon 

 is retained as material in their construction, tbe oxygen is exhaled by 

 tbe leaves and again consumed by the animal creation. Thus plants 

 prepare for man and beast the oxygen required by them, and these in 

 return give back their requisite carbonic acid. This circuit, which con- 

 ditions the life of plants and animals, extends not only to oxygen, 

 but is a general one, as first shown by Liebig (18-fO-'4G), as plants sup- 

 ply to animals the required combustible matter, in the shape of food, 

 wbile tbe animals contribute to plants the products of combustion. But 

 as in this circular course the heat of animals passes off externally, and 

 the mechanical work performed is not employed in the needful decoru- 

 po«ition of those products of oxidation, we must seek the source of 

 energy which again resolves them into their chemical components. We 

 find this in the sun, which, by means of its rays, can, according to cir- 

 cumstances, either produce or dissolve chemical combinations. We have 

 previously indicated that the sun's rays decompose the components of 

 the carbonic acid consumed by plants, and in this wise produce oxygen 

 for the inhalation of animals. 



The energy of radiation emanating from the sun is in part laid up in the 

 plaids as potential, and partly it is changed to the kinetic energy re- 

 quired in meteorological events. The latter transformation serves again 

 for the benefit of plants through the meteorological circulation of water. 

 As plants serve animals and men for food (directly, or, in the subsistence 

 on meat, indirectly), and tbe animal excretions are in due course made 

 use of by vegetation, it is apparent that ultimately the sun is the pre- 

 serving principle of tbe organic creation. On tbe existence of the sun 

 depends (lie constant preservation of tbe circular course of the terres- 

 trial organisms ; and as tbe beat supply of tbe sun is assured for at least 

 millions of years, tbe constant renewal of terrestrial organisms is guar- 

 anteed for a very long period. 



The theory of lieat lias thus led us to the discovery of the mechanical 

 equivalent of beat, and this again to the theory of the conservation 

 of energy. Guided by Ibis theory we have passed over several of the 

 principal departments in physics; we have nowhere come upon a contra- 

 diction; everywhere previously isolated phenomena have been united in 



