CORRELATION OF VITAL AND PHYSICAL FORCES. 157 



or liquid to the gaseous condition — from the visible to the invisible — 

 and that amid all these changes the same quantity of matter remains. 

 Creation or destruction of matter, increase or diminution of matter, 

 lies beyond the domain of Science ; her domain is confined entirely to 

 the changes of matter. Now, it is the doctrine of modern science that 

 the same is true of force. Force seems often to be annihilated. Two 

 cannon-balls of equal size and velocity meet each other and fall mo- 

 tionless. The immense energy of these moving bodies seems to pass 

 out of existence. But not so ; it is changed into heat, and the exact 

 amount of heat may be calculated ; moreover, an equal amount of heat 

 may be changed back again into an equal amount of momentum. 

 Here, therefore, force is not lost, but is changed from a visible to an 

 invisible form. Motion is changed from bodily motion into molecular 

 motion. Thus heat, light, electricity, magnetism, chemical affinity, 

 and mechanical force, are transmutable into each other, back and forth ; 

 but, amid all these changes, the amount of force remains unchanged. 

 Force is incapable of destruction, except by the same power which 

 created it. The domain of Science lies- within the limits of these 

 changes — creation and annihilation lie outside of her domain. 



The mutual convertibility of forces into each other is called corre- 

 lation of forces ; the persistence of the same amount, amid all these 

 protean forms, is called conservation of force. 



The correlation of physical forces with each other and with chemi- 

 cal force is now universally acknowledged and somewhat clearly con- 

 ceived. The correlation of vital force with these is not universally 

 acknowledged, and, where acknowledged, is only imperfectly conceived. 

 In 1859 I published a paper 1 in which I attempted to put the idea of 

 correlation of vital force with chemical and physical forces in a more 

 definite and scientific form. The views expressed in that paper have 

 been generally adopted by physiologists. Since the publication of the 

 paper referred to, the subject has lain in my mind, and grown at least 

 somewhat. I propose, therefore, now to reembody my views in a more 

 popular form, with such additions as have occurred to me since. 



There are four planes of material existence, which may be repre- 

 sented as raised one above another. These are : 1. The plane of ele- 

 mentary existence ; 2. The plane of chemical compounds, or mineral 

 kingdom ; 3. The plane of vegetable existence ; and, 4. The plane of 

 animal existence. Their relations to each other are truly expressed by 

 writing them one above the other, thus : 



4. Animal Kingdom. 

 3. Vegetable Kingdom. 

 2. Mineral Kingdom. 

 1. Elements. 



1 American Journal of Science, November, 1859. Philadelphia Magazine, vol. xix., 

 p. 133. 



