268 Mr. G. H. Livens : Influence of Density on Position 



the part of the curve EK of fig. 4, and postulating local 

 deviations from the mean concentration ; then a simple 

 explanation of these experiments is found in the fact that 

 pressure-rarefaction waves * of an amplitude amounting, 

 perhaps, to many atmospheres, travelling outwards from the 

 point of impact, are set up in the solution; and these waves, 

 when they pass over the local aggregations, will (if the 

 pressure change is large enough) bring the latter to a point 

 of instability analogous to K, after which the events taking 

 place are similar to that already described in paragraph (18). 

 (25) It is to be noted that the effect is the same whether 

 the solubility is increased or decreased by pressure ; in the 

 first case it will be the defect of pressure in the wave which 

 is effective, and in the second it is the increase of pressure. 

 Young also found that with supercooled pure liquids the 

 same law was operative, i. e. that the liquid solidified closer 

 to the melting-point when the blow was greater. An explan- 

 ation similar to the foregoing could easily be based on the 

 assumption that there is no such thing as a pure liquid, the 

 so-called pure liquid is only a liquid phase of very high 

 concentration ; but this far-fetched hypothesis is probably 

 not necessary, the known effect of pressure on the melting- 

 point in conjunction with an assumed want of homogeneity 

 in the liquid being sufficient to explain the phenomena. 



I wish here to express my obligations to the gentlemen in 

 my laboratory for suggestions and effective criticism, and 

 especially to Dr. Burton for working out the two thermo- 

 dynamic cycles and for other physico-mathematical help. 



XXI. The Influence of Density on the Position of the Emission 

 and Absorption Lines in a Gas-spectrum. By G. H. 



\ Livens, B.A., Fellow of Jesus College^. 



Introduction. 



rt^HE subject to be specifically discussed in this paper is 

 X the change of position of the lines in the emission and 

 absorption spectra of a gas due to changes in its density. 

 Certain effects on intensity will also be dealt with in the 

 section on absorption spectra. 



* The waves will be accompanied by small thermodynamic heating 

 and cooling effects which, in the case of the majority of aqueous solutions, 

 will act in the same direction as the corresponding pressure changes. 



f Communicated by Prof. Sir J. J. Thomson, O.M., F.R.S., being the 

 major portion of an essay which was awarded the first Smith's Prize in 

 the University of Cambridge in March 1911, 



