52 DUBLIN UNIVERSITY 



re-action are always equal and opposite ; and Dr. "Wood has laid down 

 the law, which I believe has been fully justified by his experiment, — that 

 whatever may be the amount of heat produced by the chemical action 

 of two substances when combining together, an equal amount of heat 

 would be absorbed (or cold produced), by their separation. In some in- 

 organic combinations, chemists were aware of this, as in the case of 

 the decomposition of oxide of silver by light giving rise to a depression 

 of temperature ; but the above proposition was, I believe, first proved 

 by Dr. Wood's experiments. Now, what are the changes which take 

 place in the formation of vegetable tissue ? For the most part, cellulose 

 is formed, sugar, too, and oxygen, are produced ; carbonic acid is decom- 

 posed; and water, also, bears a part. 



Carbonic acid, we know, is never formed from its elements- without 

 the production of heat ; therefore, its decomposition must produce an 

 equal amount of cold. A well-known authority in chemistry concludes 

 — "that in every part of the vegetable products of acids, neutral bo- 

 dies, cellulose, starch, fats, oils, resins, bases, and sanguineous mat- 

 ters, one character universally prevails — that, namely, of the deoxygina- 

 tion of the materials, and the liberation of oxygen." 



Some part of the oxygen is supposed to arise by the decomposition 

 of water ; and we know that the union of oxygen and hydrogen gives 

 rise to intense combustion ; in this case, also, its liberation from water 

 would exert a cooling power of great intensity. Prom experiments 

 by Grassi, we know that 1 grain of hydrogen evolved, in combus- 

 tion, 347 units of heat ; 1 grain of carbon, 79 units of heat. Desprets 

 also proved that 1 grain of oxygen, uniting with hydrogen, would raise 

 5310 grains of water one degree; and that 1 grain of oxygen uniting 

 with carbon, to form carbonic acid, would raise nearly 4000 grains of 

 water one degree ; hence, if water and carbonic acid were decomposed, 

 what a large amount of cooling effect would result ! 



The chemical changes that take place in the growth of plants are 

 generally laid down as follows : — 



12C0 2 + 10HO = C 12 H 10 O 10 + 240, 



from which we might infer, that every grain of carbonic acid decom- 

 posed would lower 4000 grains of water one degree. 

 In the formation of pertine we have — 



64 (C0 2 ) f 48HO = C^H^Oo* + 1 120 ; 



