NATIONAL ACADkMY OF SCIENI E8. 



carbonic acid from the air. The sunlight effects the decomposition 



of this, changing it to :i mucilaginous solution. On the principle 

 now indicated the water will drive the mucilaginous solution before 

 it, forcing it hack along its proper vessels into the stem. The How 

 of the. sap in plants therefore is controlled by sunlight, since this 

 agent determines the production of the mucilaginous solution, which 

 is the motive power. When the season advances the How slackens, 

 because the formation of the elaborated sap diminishes. Both in 

 the rootlet and in the leaf consequently the action is due to the 

 fact that two different liquids are brought in contact with a poroui 

 solid, which is wetted by both of them, but unequally. Hence that 

 one which has the greatest affinity for the solid and wets it most 

 perfectly passes most rapidly through it and drives the other one 

 before it. The descent of the elaborated sap is therefore quite as 

 positive an action as the ascent of the unelaborated. 



In animals the blood in the arterial capillaries of the systemic 

 circulation is charged with oxygen, which has an intense affinity for 

 the carbon and hydrogen of the walls. In the venous capillaries 

 the blood is charged with carbonic acid having no affinity for these 

 tissues. The arterial blood will drive the venous blood before it, 

 therefore. Jn the pulmonic system the venous blood is presented to 

 the air cells, for the oxygen in which it has a strong affinity, while 

 the arterial blood which has absorbed this oxygen has no longer 

 any. Movement ensues as before, but as now the affinities are 

 reversed the flow is from the veins to the arteries. The systemic 

 circulation is due therefore to the oxidizing action of the arterial 

 blood and the flow is from the artery to the vein. The pulmonary 

 circulation is due to the oxidation of the venous blood and the flow 

 is from the venous to the arterial side. Both arise from the common 

 principle that "a pressure will always be exerted by the fluid which 

 is ready to undergo a change upon that which has already under- 

 gone it a pressure which, as there is DO force to resist it, will 

 always give rise to motion in a direction from the changing to the 

 changed liquid." 



"A true theory," says Dr. Draper, " is like a window of crystal 



glass, through which we 0:111 see all objects in their proper positions 

 and colors and relations, no matter whether they are such as are 

 near or those that are at a distance ; no matter whether they are 

 directly before us or enter only obliquely into the field of view. 

 A fictitious theory is like a Venetian blind, which has to be set in a 



860 



