154 ADHESION. 



of thread is fixed with a small piece of wax to the top of th( 

 cylinder, and wound round like a screw-line, the thread: 

 being at a distance of about l cm from one another ; the other end o 

 the thread is fixed to the bottom of the cylinder. A line is thei 

 drawn with pen and ink in the middle between the lines of th< 

 thread, forming another screw-line round the cylinder. When th< 

 ink is dry, the thread is removed ; a scratch of the file is made a 

 the edge of the cylinder, and a crack is led with a pastille all alon 

 the inked lines ; or better still, the spiral may be cut between these 

 in the place previously occupied by the thread, for the crack canno 

 be well seen upon the ink. The glass should be quite dry, and th 

 pen dipped but little into the ink, or the ink will run. It is no 

 easy to carry the crack right through from one end of the cylinde 

 to the other ; but this is not necessary for our purpose. 



18. Adhesion. In the preceding article it has beei 

 stated, that if once cohesion is destroyed the particle 

 will not again manifest mutual attraction if brought ii 

 contact. Nevertheless, if the surfaces of two bodie 

 be prepared in a manner which permits their clos 

 contact if placed one upon another, mutual attractio 

 will manifest itself which may even cause the sin 

 faces to stick together. This kind of attraction is calle 

 Adhesion. 



Experiments on the force of adhesion are best mad 

 with plates of various substances, as marble, glass, meta 

 etc., with polished and perfectly flat surfaces. If to 

 such surfaces, after being carefully cleaned from dus 

 be placed upon one another by sliding one plate ov( 

 the other with a gentle pressure, a certain amount 

 force will be necessary to separate them, and this for< 

 will be greater the flatter the surfaces are, that i 

 the greater the number of points at which they ai 

 in close contact. If such plates are very accurate] 

 polished and planed, a very considerable force will 1 

 required for their separation, while even such compar; 



