22 



SIX LECTURES ON LIGHT. 



A moment's further devotion to the crys- 

 tallization of water will be well repaid ; for 

 the sum of qualities which renders ;his sub- 

 stance fitted to play its part in Nature may 

 well excite wonder and stimulate thought. 

 Like almost all other substances, water is ex- 

 panded by heat and contracted by cold. Let 

 this expansion and contraction be first illus- 

 trated : 



A small fla 1: is filled with colored water, 

 and stopped with a cork. Through the cork 

 passes a glass tube water-tight, the liquid 

 standing at a Certain height (/', Fig. 7) in the 

 tube. The flask and its tube resemble the 

 bulb and stem of a thermometer. Applying 

 the heat of a spirit-lamp, the water rises in 

 the tube, and finally trickles over the top (/). 

 Expansion by heat is thus illustrated. 



the definite temperature of 39 Fahr. Crys- 

 tallization has virtually here commenced, the 

 molecules preparing themselves for the subse- 

 quent act of solidification which occurs at 

 32, and in which the expansion suddenly 

 culminates. In virtue of this expansion, 

 ice, as you know, is lighter than water in the 

 proportion of 8 to 9.* 



It is my desire, in these lectures, to lead 

 you as closely a* possible to trie limits 

 hitherto attained by scientific thought, and, 

 in pursuance rf this desire, I have now to 

 invite your attention to a molecular problem 

 of great interest, but of great complexity. I 

 wish you to obtain sue i an insight of the 

 molecular world as shall give the intellect 

 satisfaction when reflecting on the deport- 

 n:ent of water before and during the act of 



FIG. 7. 



Projection of experiment : E is the nozzle of the lamp, L a converging lens, and / i the image of the liquid 

 column. 



Removing the lamp and piling a freezing 

 mixture in the vessel (B) round the flask, the 

 liquid column falls, thus - showing the con- 

 traction of the water by the cold. But let 

 the- freezing mixture continue to act : the 

 tailing of the column continues to a certain 

 point ; it then ceases. The top of tne col- 

 umn remains stationary for son-e seconds, 

 and afterwards begins to rise. The contrac- 

 tion has ceased, and expansion by cold sets in. 

 Let the expansion continue till the liquid 

 trickles a second time over the top of the 

 tube. The freezing mixture has here pro- 

 duced to all appearance the same effect as the 

 flame. In the case of water, contraction by 

 cold ceases and expansion by cold sets in at 



crystallization. Consider, then, the ideal 

 case of a number of magnets deprived of 



* 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 Arm- 

 strong, I had repeated opportunities of witnessing 

 this fact in his works at Elswick, in 1863. Faraday, 

 I remember, spoke to me subsequently of the com- 

 pleteness of iron castings as probably due to the 

 swelling of the metal on solidification. Beyond this, 

 I have giv^n 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 re- 

 pulsion. Certain flies walk freely upon water in vir- 

 tue of an action of this kind. With bismuth, how- 

 ever, it is easy to burst iron bottles by the force of 

 solidification. 



