Source of Motions upon the Earth. 307 



subject in different regions of the earth, and at different times and 

 circumstances in the same region. It is scarcely the place, however, 

 for entering into the discussion of this subject, and I shall only, 

 therefore, refer to it very shortly. 



It has been seen that, in different regions of the globe, the vital, 

 chemical, solar, and other forces exist or act in various proportions 

 to each other. In different seasons of the year also, and in the same 

 place, their I'elative activity varies, as we see from the diversities in 

 the actions of vegetable life in the different seasons ; and it is more- 

 over certain that, in the same plaCe, their relative activity varies 

 from year to year, of which we have an illustration in the fact that 

 the vegetation and the harvests of no two years are perfectly alike. 

 The growth and the maintenance of the body, — all those motions, 

 changes, and arrangements of the molecules of the matter of which 

 it is made up, and in which life consists, — are the result of the action 

 of these forces. Certain changes and arrangements among the mole- 

 cules produce a condition of the body to which the term normal or 

 healthy is applied ; and the various forces we have spoken of, exist- 

 ing, and acting in a certain relative proportion to each other, cause 

 the molecular motions and arrangements which give i-ise to this con- 

 dition. Again, certain motions, changes, and arrangements of the 

 molecules of the different matters of which the body consists, produce 

 conditions of the body to which the term abnormal, or unhealthy, is 

 applied. These, as before, must result from the action of the same 

 force, in certain relative proportions, and the conditions may be in- 

 finite in number. When the state of the body and all its actions, 

 or, in other woi'ds, when the vital forces are adapted to and in con- 

 cordance with the external forces, we have the condition of health ; 

 when they are not so, we have disease. In different regions of the 

 earth, therefore, the conditions of health will result from different 

 proportions between the several forces. That proportion between the 

 forces which in this country is productive of health, may in another, 

 a tropical country for example, produce disease. A European going 

 to a tropical country, carries with him in ordinary circumstances 

 a European constitution of body, formed by, and dependent upon, 

 the relative activity and proportion of the various forces which pre- 

 vail in his native place. The same degree of activity, and the same 

 proportion, does not hold in the warm climate ; his constitution is not 

 in accoi'danco with it, and, we should therefore expect that disease 

 will result. 



We accordingly find that inhabitants of a cold or a temperate cli- 

 mate, on removing to a tropical country, are subject to many diseases 

 in a degree to which the native inhabitants are not ; and that on 

 their arrival they are liable to " seasoning fevers," as they are called, or 

 to something approaching to them, in which it may be supposed 

 that the opposing forces seek to equilibriate themselves, or that 



