244 Mr. C. V. Boys on Water-pipes 



tive index in the red. We may say that we have a hint of 

 this; for if we represent the refractive indices by the ordinates 

 of a curve in which the squares of the reciprocals of the 

 wave-lengths are abscissas, this curve presents a point of 

 inflection*. In the part corresponding to short waves it is 

 concave upwards ; in the part corresponding to long waves 

 it is concave downwards : the curvature, however, is very 

 slight. Does it not seem possible, looking at the matter from 

 the purely optical point of view, that if we could examine the 

 spectrum below the absorption in the infra-red, we should find 

 the effect of anomalous dispersion, and that the refractive index 

 of such long waves might even be so high as 2*6? To test 

 this experimentally in a conclusive manner would probably 

 not be easy. Perhaps the best chance of finding how these 

 long waves are refracted would be to experiment on the rays 

 from a thermopile to a freezing-mixture. Without an actual 

 measurement of a refractive index below all strong absorption, 

 it cannot be said that experiment is in contradiction to the 

 Electromagnetic Theory of Light; for a strong absorption 

 introduces a discontinuity into the spectrum which forbids us 

 from using results on one side of that discontinuity to infer 

 what they would be on the other side. 



XXX. Water-pipes that do not burst with Frost. By C. 

 Vernon Boys, Demonstrator of Physics, Normal School of 

 Science, South Kensington]. 



DURING the severe weather of last winter, Mr. L. S. 

 Powell proposed to me a scheme for preventing the 

 possibility of water-pipes bursting through frost ; and I have 

 since learnt that Mr. Mangnall, of Manchester, independently 

 hit upon the same idea. As far as I can remember, there 

 were some letters in the ( Times ' describing the use of india- 

 rubber pipes containing air inserted in the service-pipes. This 

 would obviously prevent pipes from bursting; for the pressure 

 is of a nature that is relieved by a comparatively small expan- 

 sion; and this the india-rubber tube allows to take place in the 

 surrounding water when it collapses. There is, however, one 

 serious objection to this, which is the possibility of the detach- 

 ment of one end of the flexible tube, in which case a rush of 

 water might cause it to accumulate in one place and obstruct 

 the passage. 



* Proceedings of Koyal Society, 1877. 



t Communicated by the Physical Society, having been read at the 

 Meeting on November 12, 1881. 



