THE RADIOMETER. 3 



Galileo encountered more than half a century later, were due solely to 

 the circumstance that the new theory tended to subvert the popular 

 faith in the cosmography of the Church. In modern times, with the 

 many popular expositors of science, the diffusion of new truth is more 

 rapid ; but even now there is always a long interval after any great 

 discovery in abstract science before the new conception is translated 

 into the language of common life, so that it can be apprehended by the 

 mass even of educated men. 



I have thus dwelt on what must be familiar facts in the past history 

 of astronomy, because they illustrate and will help you to realize the 

 present condition of a much younger branch of physical science : for in 

 the transition-period I have described there exists now a conception 

 which opens a vision into the microcosmos beneath us as extensive and 

 as grand as that which the Copernican theory revealed into the macro- 

 cosmos above us. 



The conception to which I refer will be at once suggested to every 

 scientific scholar by the word molecule. This word is a Latin diminu- 

 tive, which means, primarily, a small mass of matter ; and although 

 heretofore often applied in mechanics to the indefinitely small particles 

 of a body between which the attractive Or repulsive forces might be 

 supposed to act, it has onty recently acquired the exact significance 

 with which we now use it. 



In attempting to discover the original usage of the word molecule, 

 I was surprised to find that it was apparently first introduced into 

 science by the great French naturalist, Buffon, who employed the term 

 in a very peculiar sense. Buffon does not seem to have been troubled 

 with the problem which so engrosses our modern naturalists how the 

 vegetable and animal kingdoms were developed into their present con- 

 dition but he was greatly exercised by an equally difficult problem, 

 which seems to have been lost sight of in the present controversy, and 

 which is just as obscure to-day as it was in Buffon's time, at the close 

 of the last century, and that is, Why species are so persistent in 

 Nature ; why the acorn always grows into the oak, and why every 

 creature always produces of its kind. And, if you will reflect upon it, 

 I am sure you will conclude that this last is by far the more fundamen- 

 tal problem of the two, and one which necessarily includes the first. 

 That of two eggs, in which no anatomist can discover any structural 

 difference, the one should, in a few short years, develop an intelligence 

 like Newton's, while the other soon ends in a Guinea-pig, is certainly a 

 greater mystery than that, in the course of unnumbered ages, monkeys, 

 by insensible gradations, should grow into men. 



In order to explain the remarkable constancy of species, Buffon ad- 

 vanced a theory which, when freed from a good deal that was fanciful, 

 may be expressed thus : The attributes of every species, whether of 

 plants or of animals, reside in their ultimate particles, or, to use a more 

 philosophical but less familiar word, inhere in these particles, which 



