74 I'llOl'EKTIES OF SALINE SOLUTIONS — MACGREGOR. 



flame be applied as ])et'ore, tlie yellow solution is at once seen to 

 become orange and the orange solution red. Owing to the nar- 

 rowness of the trough and the thinness of its glass sides, sufficient 

 heating to produce a marked change of colour occupies only half 

 a minute or so. The same trough may of course be used to pro- 

 ject the absorption spectra of these solutions on the screen. If 

 the slit l)e covered half by the one solution and half by the 

 other, l:)(_)th absorption spectra may be seen at once, side by side,, 

 and the gradual variation of the spectra may be watched as tlie 

 trough is gradually heated. 



As a means of showing the variation of the colour or al (sorp- 

 tion spectrum of a solution with concentration, the above experi- 

 ment has an obvious defect, viz., that the thickness of the layer 

 of the strong and weak solutions being ecjual, the numbers (jf 

 the salt molecules through which any ray of light passes are 

 very different in the two cases. It should therefore be supple- 

 mented by showing also the colour or the spectrum obtained 

 when the light is passed through a wide trough of the dilute 

 solution, the ratio of the widths of the troug-hs beinp- the reci- 

 procal of the ratio of the percentages of salt in the two solutions.. 



(3.) Dr. W. W. J. Xicol's observation* that anhydrous sodium 

 sulphate will dissolve in a supersaturated solution of that salt 

 may readily be shown as a lecture experiment by projection. 

 For that purpose place a test tuV)e containing the solution, in a 

 troucfh with o-lass sides full of water, and focus it on the screen. 

 Then, let the anhydrous salt in the form of a tine powder, fall 

 upon the surface of the solution. By taking a pinch of the 

 powder between the thumb and forefinger (both being quite 

 dry), it may be made to fall as a shower of tine particles. 

 These pass into the solution and are seen to move slowly across 

 the screen through the solution, dissolving as they go, in some 

 cases disappearing, and often changing the concentration of the 

 part of the solution through which they have passed, so as to 

 produce obvious refraction effects. Finally, to show that the 

 solution was supersaturated, add a few crystals of the hydrated 

 salt and crystallization at once occurs. The anhydrous salt/ 



* rhil. Mag., i^er. 5, Vol. xix (lS»5)p. 4.53. 



