72 PROPERTIES OF SALINE SOLUTIOXS — MACGllEGOll. 



volume may be indicated hy a comparati\"ely large e]e^■ati^)ll or 

 depression in the tube. The hydroxide should ])e in the form of 

 a powder, not only that its solution may 1 )e accomplished quickly, 

 hut also l;)ecause the solution formed must be dilute in order to 

 secure a depression of the li(|uid in the tulie. If it l»e not pow- 

 dered, the substance falls to the bottom and forms a strong solu- 

 tion there, which only gradually diffuses into the water aljove. 

 Even with a fine powder, however, a comparatively strong solu- 

 tion is formed at the Ijottom. Hence I have found it advisable 

 to catch the powder in a wire gauze cage, attached by sealing- 

 wax to the inner end of the rubl)er stopper which carries the 

 tube, and to hasten the mixture of the strong solution, formed in 

 the tube and cage, with the water, by diverting the downward 

 currents of the strong solution towards the sides of the bottle by 

 means of a plate of glass hanging horizontally below the cage. 

 If a ^^'ide-mouthed bottle he used, a stirrer may be introduced 

 through the stopper, but leakage is thereby rendered more prob- 

 able. The full amount of the contraction indicated by Kohl- 

 rauscli's observations cannot, of course, he shown. For (1), the 

 powdered caustic soda already contains a considerable quantity 

 of water; (2), the solution of the substance is attended lyy a 

 development of heat involving a rise of the liquid in the tube ; 

 (3), the powder carries air with it into the water, which must 

 increase the volume whether it dissolves or remains suspended ; 

 for in the latter case, if a quick effect is desired, there is not suf- 

 ficient time for it to escape up the tube ; and (4), whatever pre- 

 cautions may be taken to secure a uniform solution throughout 

 the bottle, it cannot be at all completeh' secured in the 

 time at disposal. But notwithstanding these difficulties, the 

 experiment is a ver}' striking one, especially if projected by 

 a lantern on a screen. As the powdered caustic soda is passed 

 down the tube, little by little, the liquid is seen to dissolve it 

 without any increase in bulk, and if the substance does not 

 already contain too much water, with an actual diminution in 

 Inilk, the level of the li(juid sinking in the tube. If the powder 

 be added in large quantit}-, there is a sudden rise of liquid in the 

 tube, followed bv a gradual shrinkage, which continues until the 



