August 14, 1891.] 



SCIENCE. 



9* 



I referred, when I began these jottings, to the idea that the 

 explanation of a trick was to be found in the delusion of 

 the spectator's senses; and this reminds me of a very inter- 

 esting case which certainly proves to us how a dominant 

 idea may be impressed on the minds not of a few spectators 

 but of thousands, with an utterly futile result when all is 

 said and done. When the Crystal Palace took fire, many 

 years ago, efforts were made to rescue the animals from the 

 menagerie, which was lodged in the burning part. As the 

 fire progressed, a large monkey was seen by the spectators to 

 appear on the roof and to hold on to some pinnacle or other, 

 apparently writhing in terror at its impending incineration. 

 Desperate attempts to reach the unfortunate animal were 

 made. The crowd was breathless with anxiety. Every 

 movement of the rescuers was watched with agonizing inter- 

 est. At last the ape was reached, and was found to be — a 

 piece of canvas, which had apparently been detached from 

 the building, and which, clinging to some post or pole, had 

 impressed the crowd, by its flapping, with the idea that it 

 was a big monkey writhing with fear and agony. This, I 

 believe, isa well-founded fact. It proves to the full, of course, 

 that, given an idea, supported by a fair show of demonstra- 

 tion, such a thought is certain to become dominant and over- 

 ruling in the minds of many men. How far this principle 

 may serve to explain many another delusion of human life, 

 I leave my readers to judge. 



LECTURE EXPERIMENTS ILLUSTRATING PROPERTIES 

 OF SALINE SOLUTIONS.' 



(1) In a paper printed in the last volume of this institute's pro- 

 ceedings (Proo. N. S. Inst. Nat Sci., vii., 368) I pointed out 

 that, according to Kohlrausch's observations, suflBciently dilute 

 solutions of sodium hydroxide have volumes which are less than 

 the volumes which their solvent water would have in the free 

 state, one gram of a solution containing about six per cent of the 

 hydroxide, having a volume 0.0045 of a cubic centimetre less than 

 the water contains. Several other substances are known which 

 exhibit the phenomenon of contraction on solution, in a similarly 

 marked manner, but none which exhibit it to such an extent. This 

 hydroxide, therefore, affords the best means of exhibiting the 

 phenomenon of contraction by a lecture experiment. 



The simplest mode of conducting the experiment is to pass the 

 powdered caustic soda, little by little, down a glass tube forming 

 a prolongation of the neck of a large bottle, the bottle and part of 

 the tube having been first tilled with distilled (or, indeed, undis- 

 tilled) water. The substance is quickly dissolved by the water, 

 the strong solution thus formed sinks and mixes with the water 

 below, and the change of volume of the liquid is indicated by the 

 change of height of the column of liquid in the tube. In order 

 that the experiment may be made quickly, the powder must not 

 be allowed to form a cake in the tube where it meets the water. 

 To avoid this, a tube of about seven or eight millimetres in diam- 

 eter must be used. It should be seven inclies in length, and should 

 have the upper end opened out to a funnel shape, to facilitate the 

 introduction of the powder. The tube being necessarily of large 

 bore, the bottle must also be large, so that a small change of vol- 

 ume may be indicated by a comparatively large elevation or de- 

 pression in the tube. 



The hydroxide should be in the form of a powder, not only that 

 its solution may be accomplished quickly, but also because the 

 solution formed must be dilute in order to secure a depression 

 of the liquid in the tube. If it be not powdered, the substance 

 falls to the bottom and forms a strong solution there, which only 

 gradually difiuses into the water above. Even with a fine powder, 

 however, a comparatively strong solution is formed at the bottom. 

 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 rubber 



^ Professor J. G. MacGregor, in Transactions of Nova Scotian Institute of 

 Science, session of 1890-91. 



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 downvpard currents of the strong solution towards 

 the sides of the bottle by means of a plate of glass hanging hori- 

 zontally below the cage. If a wide-mouthed bottle be used, a 

 stirrer may be introduced through the stopper, but leakage is 

 thereby rendered more probable. 



The full amount of the contraction indicated by Kohlrausch's 

 observations cannot, of course, be shown. For (a) the powdered 

 caustic soda already contains a considerable quantity of water ; (6) 

 the solution of the substance is attended by a development of beat 

 involving a rise of the liquid in the tube; (c) 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 sufBcient time for it to escape up the 

 tube; and (d) whatever precautions may be taken to secure a uni- 

 form solution throughout the bottle, it cannot be at all completely 

 secured in the time at disposal. But notwithstanding these difiS- 

 culties, the experiment is a very 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 with- 

 out any increase in bulk, and if the substance does not already 

 contain too much water, with an actual diminution in bulk, the 

 level of the liquid sinking in the tube. If the powder be added in 

 large quantity, there is a sudden rise of liquid in the tube, fol- 

 lowed by a gradual shrinkage, which continues until the level of 

 the liquid is lower than at the outset. The amount of the de- 

 pression of the liquid in the tube is sometimes small, depending 

 apparently upon the amount of water which the powdered caustic 

 soda has already absorbed. The substance should not be too finely 

 powdered, as in that case it is likely both to have taken up a con- 

 siderable quantity of water and to carry down with it a considerable 

 quantity of air. The experiment requires only a few minutes to 

 perform. 



(2) The working hypothesis which I use when thinking of the 

 phenomena of solution, has led me to the conclusion that eleva- 

 tion of the temperature of a solution will have, if not identically, 

 at any rate in a general way, the same effect on its selective ab- 

 sorption of light, and therefore on its color, as increase in its 

 concentration. All the experimental evidence of which I can find 

 any record bears out this conclusion. But, whether it holds gen- 

 erally or not, it may be shown, by a striking lecture experiment, 

 to hold in the case of two salts, the chlorides of cobalt (C0CI2) and 

 of iron (FeCl,.). To do so, make a trough, for projection with a 

 lantern, having thin glass sides, about the size of a lantern-slide, 

 the gla.'is sides being one or two millimetres from one another. 

 It may readily be made by cutting a U-shaped piece from a sheet 

 of India-rubber, and cementing the glass plates to its opposite 

 sides. Half fill the trough with a saturated solution of either salt, 

 and fill up with a weak solution. If cobalt chloride have been 

 used, the solution in the lower part of the trough will at ordinary 

 teuiperatures be of a purplish blue, that in the upper part red; 

 and it will be obvious that increase of the concentration of this 

 salt involves increase of blueness in the transmitted light. If, 

 now, a Bunsen flame be played carefully over one side of the 

 trough, the solutions rapidly rise in temperature, and both are 

 seen to increase in blueness, the saturated solution becoming deep 

 blue and the weak solution purplish red. If the iron chloride 

 have been used, the solution- in the lower part of the trough, 

 before heating, is seen to be of a deep orange color, that in the 

 upper part yellow ; and it is obvious that increase in the concen- 

 tration of this salt involves increase in redness. If, now. the 

 flame be applied as before, the 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, sufBcient 

 heating to produce a marked change of color occupies only half a 

 minute or so. The same trough may of course be used to project 

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

 be covered half by the one solution and half by the other, both ab- 

 sorption spectra may be seen at once, side by side, and the gradual 

 variation of the spectra may be watched as the trough is gradually 

 heated. 



As a means of showing the variation of the color or absorption 



