224 ADDRESS TO THE MATHEMATICAL AND PHYSICAL SECTIONS 



cannot fail to disturb the commonly received opinion that a molecule, in order 

 to be permanent, must be a very hard body. 



In fact one of the first conditions which a molecule must fulfil is, ap- 

 parently, inconsistent with its being a single hard body. We know from those 

 spectroscopic researches which have thrown so much light on different branches 

 of science, that a molecule can be set into a state of internal vibration, in 

 which it gives off to the surrounding medium light of definite refrangibility 

 light, that is, of definite wave-length and definite period of vibration. The 

 fact that all the molecules (say, of hydrogen) which we can procure for our 

 experiments, when agitated by heat or by the passage of an electric spark, 

 vibrate precisely in the same periodic time, or, to speak more accurately, that 

 their vibrations are composed of a system of simple vibrations having always 

 the same periods, is a very remarkable fact. 



I must leave it to others to describe the progress of that splendid series 

 of spectroscopic discoveries by which the chemistry of the heavenly bodies has 

 been brought within the range of human inquiry. I wish rather to direct 

 your attention to the fact that, not only has every molecule of terrestrial 

 hydrogen the same system of periods of free vibration, but that the spectro- 

 scopic examination of the light of the sun and stars shews that, in regions 

 the distance of which we can only feebly imagine, there are molecules vibrating 

 in as exact unison with the molecules of terrestrial hydrogen as two tuning- 

 forks tuned to concert pitch, or two watches regulated to solar time. 



Now this absolute equality in the magnitude of quantities, occurring in all 

 parts of the universe, is worth our consideration. 



The dimensions of individual natural bodies are either quite indeterminate, 

 as in the case of planets, stones, trees, &c., or they vary within moderate 

 limits, as in the case of seeds, eggs, &c. ; but even in these cases small quanti- 

 tative differences are met with which do not interfere with the essential 

 properties of the body. 



Even crystals, which are so definite in geometrical form, are variable with 

 respect to their absolute dimensions. 



Among the works of man we sometimes find a certain degree of uniformity. 



There is a uniformity among the different bullets which are cast in the 

 same mould, and the different copies of a book printed from the same type. 



If we examine the coins, or the weights and measures, of a civilized 

 country, we find a uniformity, which is produced by careful adjustment to 



