THE 



POPULAR SCIENCE 

 MONTHLY. 



JANUARY, 1875. 



CRYSTALLINE AND MOLECULAR FORCES. 1 



Br Prof. JOHN TYNDALL, LL. D., F. E. S. 



A FEW years ago I paid a visit to a large school in the country, 

 and was asked by the principal to give a lesson to one of his 

 classes. I agreed to do so, provided he would let me have the young- 

 est boys in his school. To this he willingly assented ; and, after cast- 

 ing about in my mind as to what could be said to the little fellows, I 

 went to a village hard by and bought a quantity of sugar-candy. This 

 was my only teaching apparatus. When the time for assembling the 

 class had arrived I began by describing the way in which sugar-candy 

 and other artificial crystals were formed, and tried to place vividly 

 before their young minds the architectural process by which the crys- 

 tals were built up. They listened to me with the most eager interest. 

 I examined the crystal before them, and, when they found that in a 

 certain direction it could be split into thin larninse with shining sur- 

 faces of cleavage, their joy was at its height. They had no notion 

 that the thing they had been crunching and sucking all their lives 

 embraced so many hidden points of beauty. At the end of the lesson 

 I emptied my pockets among the class, and permitted them to experi- 

 ment upon the sugar-candy in the usual way. 



When asked to come here and lend a helping hand in what I be- 

 lieve to be a truly good work (though hard pressed by other duties), 

 I could not refuse the invitation. 



I kuow not whether this great assembly will deem it an imperti- 

 nence on my part if I seek to instruct them for an hour or so on the 

 subject chosen for my little boys. In doing so I run the imminent 

 risk of being wearisome as well as impertinent, while laboring under 

 the further disadvantage of not being able to make matters pleasant 

 at the conclusion of the lecture by the process adopted at the end of 

 my lesson to the boys. 



1 A Lecture, delivered in the Free-Trade Hall, Manchester, on Wednesday, October 

 28, 1874. 



VOL. VI. 11 



