10 WRITINGS OF JOSEPH HENRY. [1855- 



acters, namely, the lighting impulse, the heating impulse, the 

 chemical impulse, and the phosphorogenic impulse; and it 

 has further been ascertained that though each of these im- 

 pulses may produce an effect on the plant, the decomposition 

 of the carbonic acid is mainly due to the chemical action. 

 A series of experiments is required to determine the various 

 conditions under which these impulses from the sun may 

 be turned to the greatest amount of economical use, and 

 what modifications they may demand, in order to the 

 growth of peculiar plants. It has not yet been clearly ascer- 

 tained whether some of these emanations cannot be ex- 

 cluded with beneficial result, or in other words, whether 

 they do not produce an antagonistic effect ; nor is it known 

 what relative proportions of them are absorbed by the at- 

 mosphere, or reflected from our planet by the floating clouds 

 of the air, without reaching the earth. To determine these 

 facts requires a series of elaborate experiments and accurate 

 observations. 



We have said that the chemical vibration is that which 

 principally decomposes the carbonic acid in the growth of 

 the plant ; but we know that the heating impulse is an aux- 

 iliary to this, and that heat and moisture are essential ele- 

 ments in the growth of vegetation. The small amount of 

 knowledge we already possess of the character of the emana- 

 tions from the sun has been turned to admirable account 

 in horticulture. In this branch of husbandry we seek — ■ 

 even more than in agriculture — to modify the processes of 

 nature; to cultivate the plants of the torrid zone amid the 

 chilling winds of the northern temperate zone, and to render 

 the climate of sterile portions of the earth congenial to the 

 luxurious productions of more favored regions. We seek 

 to produce artificial atmospheres, and to so temper the im- 

 pulses from the sun that the effects of variations in latitude 

 and the rigor of the climate may be obviated. 



From all that has been said therefore it will be evident 

 that the hopes of the future, in regard to agriculture, rest 

 principally upon the advance of abstract science, not upon 

 the mere accumulation of facts, of which the connection and 

 dependence are unknown, but upon a definite conception of 



