Volume Changes attending Mixture. 507 



(1) the diffusion during the second filling of the two liquids 

 against gravity, an operation which lasts about 15 minutes, 

 and (2) the imperfect purity of the substances. The first of 

 these is quite negligible, as the common surface is only about 

 1 square millimetre, while the volumes are over 3 cubic centi- 

 metres. 



In taking the tube out of its bath it is tilted, bulb upwards, 

 so as to be at an angle of about 45°. Where the differences 

 of refractive index and of density are great, complete mixture 

 seems to be reached in five or six reversals of a few minutes 

 each *. But in none of the cases given above were the 

 reversals less than 20 in number nor of less duration than 10 

 minutes each, nor was the manipulation stopped before trial 

 showed that constancy of volume had been reached. Each 

 experiment from first to last takes about five hours. The open 

 end of the capillary may be stopped by a short piece of caout- 

 chouc-tubing containing a glass plug. 



§ 269. Returning to Table LXL, a cloud of ideas suggest 

 themselves. The results are obtained on equal volumes at an 

 arbitrary temperature 17°*6 both before and after mixture. 

 Wherever there is gain of volume, there is absorption of heat, 

 cold, diminished heat-tension (C). Wherever there is loss of 

 volume, there is liberation of heat, heating, increased heat- 

 tension (H). These thermal effects are only given qualita- 

 tively in the Table, and were actually obtained in a second 

 series of experiments made as follows. Equal volumes of the 

 two liquids were run from burettes into two test-tubes of thin 

 glass, one of about twice the capacity of the other. The nar- 

 rower was put inside the wider, and both put into water at 

 17°*6. After sufficient standing, a thermometer in the inner 

 liquid marks nearly the same temperature. The tubes are 

 removed from the water and w T iped, the inner one broken 

 through at the bottom and used as a stirrer. 



Let us consider, now, two of these cases more exactly, in 

 one of which there is heat and contraction, in the other cold 

 and expansion. 



* In making suck experiments as these, tke question presents itself — 

 If two colourless (or similarly coloured) liquids have the same refractive 

 index, how shall we know whether they are mixed or not ? And if, 

 therewith, they have the same density, how does their emulsion differ 

 at all from their mixture ? It is, indeed, unlikely that any two liquids 

 have the same refractive indices or the same density at more than one 

 temperature. And that they should have both refractive index and density 

 alike at more than one temperature involves an improbability of a higher 

 order. Apart from this, however, it is likely that the skin of* a liquid has 

 a different (higher) density and a higher refractive index than the mass of 

 the same liquid. (F. G., Phil. Mag. November 1883.) 



2L2 



