8 CONTENTS. 



PAGE 



Experiments 32 



Influence of the atmospheric pressure 33 



Water passes through membranes more easily than air does 34 



Experiments on evaporation through membranes 34 



Importance of the cutaneous transpiration 35 



By it the fluids acquire a motion towards the skin and lungs 35 



Effects of dry and moist air, and of elevation, on the health 35 



Causes of the efflux of sweat 36 



Fishes die in air, because the due distribution of the fluids is prevented 36 



Experiments of HALES on the motion of the sap in plants 36 



This motion is caused by evaporation 37 



Force with which the sap rises 37 



The atmospheric pressure is the active force 38 



The sap absorbs gases 38 



The evaporation supplies food to the plant 38 



Influence of suppressed evaporation on hop vines 39 



Observations of HALES on the blight in hops, &c 39 



Fire-blasts in hops 39 



HALES recognized the influence of evaporation on the life of plants 39 



The origin of the potato disease is probably similar to that of the blight in hops. . . 40 



The disease long known 40 



It is due, not to a degeneration of the plant, but to a combination of external cir- 

 cumstances 40 



It is connected with the weather, and particularly with the temperature and hygro- 



metric state of the atmosphere 41 



The life of plants is dependent chiefly on four external causes 41 



Only one of which, namely, the quality of the soil, is in the power of the agriculturist 41 



Effects of suppressed evaporation 41 



The fungi and putrefaction follow the death of the plant 41 



Observations of HALES on the rise of the spring sap in perennial plants 41 



Views of DUTROCHET . . . t 42 



Objections to these views 42 



The cause of the rise of the sap is transient, and depends on external influences. . . 42 



It exists, not merely in the spongioles, but in all parts of the plant 42 



Experiments of HALES 43 



His conclusions 43 



Gas is given off by the sap 44 



The rise may therefore be due to the disengagement of gas 44 



The gas is probably carbonic acid 44 



APPENDIX 



Account of a plan proposed by Dr. KLOTZSCH, of Berlin, protecting potato plants 



from disease 45 



This plan published by authority of the Minister of the Interior of Prussia, on the 



favorable report of the President of the College of Kural Economy at Berlin .... 47 



Conditions on which the reward, claimed for his plan, if found effectual, by Dr. 



KLOTZSCH, has been granted , 47 



