250 



SCIENTIFIC AMUSEMENTS. 



matter contained in the pencil marks is formed, which tints the 

 paper as a soluble ink would do ; or by pressing damp tissue paper 

 on the writing a copy is obtained, the colouring matter dissolving 

 in the water moistening the tissue paper. 



In Expt. 140 directions are given for removing from paper, 

 &c., ink stains produced by inks of the gallnut and iron class. 



Expt. 295. Capillary Attraction and Repulsion. The 

 difference between the relative amounts of adhesion of various 

 liquids to given solids, and the cohesion of the liquids themselves, 

 gives rise to the difference observed in the behaviour of various 

 liquids when one and the same solid is dipped into them and 

 removed; thus a glass rod dipped into ivater or alcohol comes 



Fig. 100. Rods of Glass 

 dipped in Water. 



Fig. 101. Rods of Glass 

 dipped in Mercury. 



out wetted with the fluid ; i.e., a portion of the fluid adheres 

 to the glass so strongly that the adhesion overcomes the cohesion 

 of this part of the liquid for the rest^ so that when the rod is 

 lifted out from the liquid this portion is removed too. If, however, 

 a similar rod be dipped into clean pure mercury, no metal whatever 

 will stick to it on lifting it out, the adhesion of the mercury to 

 the glass being less than its cohesion towards the portions of 

 mercury adjoining. As a consequence of the nature of the 

 surface actions producing this difference, the surface of a basin of 

 water into which a freely suspended rod of glass (or any other 

 body wetted by the water) is dipped, will be found to be elevated 

 in a curve towards the surface of the glass, as in fig. 100 ; whilst 

 the opposite action (fig. 101) will be manifest when mercury is 

 used instead of water, or if the glass be well greased, so as not to 



