OF THE BRITISH ASSOCIATION. 225 



standards made and provided by the state. The degree of uniformity of these 

 national standards is a measure of that spirit of justice in the nation which 

 has enacted laws to regulate them and appointed officers to test them. 



This subject is one in which we, as a scientific body, take a warm 

 interest ; and you are all aware of the vast amount of scientific work which 

 has been expended, and profitably expended, in providing weights and measures 

 for commercial and scientific purposes. 



The earth has been measured as a basis for a permanent standard of 

 length, and every property of metals has been investigated to guard against 

 any alteration of the material standards when made. To weigh or measure any 

 thing with modern accuracy, requires a course of experiment and calculation 

 in which almost every branch of physics and mathematics is brought into 

 requisition. 



Yet, after all, the dimensions of our earth and its time of rotation, though, 

 relatively to our present means of comparison, very permanent, are not so by 

 any physical necessity. The earth might contract by cooling, or it might be 

 enlarged by a layer of meteorites falling on it, or its rate of revolution might 

 slowly slacken, and yet it would continue to be as much a planet as before. 



But a molecule, say of hydrogen, if either its mass or its time of vibration 

 were to be altered in the least, would no longer be a molecule of hydrogen. 



If, then, we wish to obtain standards of length, time, and mass which 

 shall be absolutely permanent, we must seek them not in the dimensions, or 

 the motion, or the mass of our planet, but in the wave-length, the period of 

 vibration, and the absolute mass of these imperishable and unalterable and 

 perfectly similar molecules. 



When we find that here, and in the starry heavens, there are innumerable 

 multitudes of little bodies of exactly the same mass, so many, and no more, 

 to the grain, and vibrating in exactly the same time, so many times, and no more, 

 in a second, and when we reflect that no power in nature can now alter in 

 the least either the mass or the period of any one of them, we seem to have 

 advanced along the path of natural knowledge to one of those points at which 

 we must accept the guidance of that faith by which we understand that 

 " that which is seen was not made of things which do appear." 



One of the most remarkable results of the progress of molecular science 

 is the light it has thrown on the nature of irreversible processes processes, 

 that is, whidh always tend towards and never away from a certain limiting 



VOL. II. 29 



