Page:Popular Science Monthly Volume 6.djvu/271

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THE

POPULAR SCIENCE

MONTHLY.

 

JANUARY, 1875


 

CRYSTALLINE AND MOLECULAR FORCES.[1]
By Prof. JOHN TYNDALL, LL. D., F. R. 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 youngest boys in his school. To this he willingly assented; and, after casting 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 crystals 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 laminæ with shining surfaces 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 experiment upon the sugar-candy in the usual way.

When asked to come here and lend a helping hand in what I believe to be a truly good work (though hard pressed by other duties), I could not refuse the invitation.

I know not whether this great assembly will deem it an impertinence 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.