334 AIM AND PROGRESS OF PHYSICAL SCIENCE. 



bution. If it disappear in one form, it reappears as surely in 

 another ; and whenever it presents itself in a new phase we are 

 certain that it does so at the expense of one of its other forms. 



Carnot's law of the mechanical theory of heat, as modified 

 by Clausius, has, in fact, made it clear that this change moves 

 in the main continuously onward in a definite direction, so that 

 a constantly increasing amount of the great store of energy in 

 the universe is being transformed into heat. 



We can, therefore, see with the mind's eye the original 

 condition of things in which the matter composing the celestial 

 bodies was still cold, and probably distributed as chaotic vapour 

 or dust through space ; we see that it must have developed heat 

 when it collected together under the influence of gravity. Even 

 at the present time Spectrum analysis (a method the theoretical 

 principles of which owe their origin to the mechanical theory 

 of heat) enables us to detect remains of this loosely distributed 

 matter in the nebulse; we recognise it in the meteor-showers 

 and comets ; the act of agglomeration and the development of 

 heat still continue, though in our portion of the stellar system 

 they have ceased to a great extent. The chief part of the 

 primordial energy of the matter belonging to our system is now 

 in the form of solar heat. This energy, however, will not 

 remain locked up in our system for ever : portions of it are 

 continually radiating from it, in the form of light and heat, 

 into infinite space. Of this radiation our earth receives a share. 

 It is these solar heat-rays which produce on the earth's surface 

 the winds and the currents of the ocean, and lift the watery 

 vapour from the tropical seas, which, distilling over hill and 

 plain, returns as springs and rivers to the sea. The solar rays 

 impart to the plant the power to separate from carbonic acid 

 and water those combustible substances which serve as food for 

 animals, and thus, in even the varied changes of organic life, 

 the moving power is derived from the infinitely vast store of 

 the universe. 



This exalted picture of the connection existing between all the 

 processes of nature has been often presented to us in recent 

 times ; it will suffice here that I direct attention to its leading 



