[15] The beautiful plumes produced by water-crystallization have been successfully photographed by Professor Lockett.
[16] In a little volume entitled 'Forms of Water,' I have mentioned that cold iron floats upon molten iron. In company with my friend Sir William Armstrong, I had repeated opportunities of witnessing this fact in his works at Elswick, 1863. Faraday, I remember, spoke to me subsequently of the perfection of iron castings as probably due to the swelling of the metal on solidification. Beyond this, I have given the subject no special attention; and I know that many intelligent iron-founders doubt the fact of expansion. It is quite possible that the solid floats because it is not wetted by the molten iron, its volume being virtually augmented by capillary repulsion. Certain flies walk freely upon water in virtue of an action of this kind. With bismuth, however, it is easy to burst iron bottles by the force of solidification.
[17] This beautiful law is usually thus expressed: The index of refraction of any substance is the tangent of its polarizing angle. With the aid of this law and an apparatus similar to that figured at page 15, we can readily determine the index of refraction of any liquid. The refracted and reflected beams being visible, they can readily be caused to inclose a right angle. The polarizing angle of the liquid may be thus found with the sharpest precision. It is then only necessary to seek out its natural tangent to obtain the index of refraction.
[18] Whewell.
[19] Removed from us since these words were written.
[20] The only essay known to me on the Undulatory Theory, from the pen of an American writer, is an excellent one by President Barnard, published in the Smithsonian Report for 1862.
[21] Boyle's Works, Birch's edition, vol. i. pp, 729 and 730.
[22] Werke, B. xxix. p. 24.
[23] Defined in Lecture I.
[24] This circumstance ought not to be lost sight of in the examination of compound spectra. Other similar instances might be cited.