ADHESION. 155 



tively rough surfaces as those obtained by cutting two 

 leaden cylinders, such as those described below (p. 158), 

 with a penknife, so as to form two even and bright sur- 

 ? aces, will, if pressed and turned against each other until 

 ;hey are in close contact, adhere so strongly as to 

 equire a force of more than 100 gr to separate them, 

 "f plates of different substances for example, glass 

 ind brass are placed in contact, they will be found to 

 nanifest adhesion as well as plates of the same sub- 

 tance for example, glass and glass. 



Various modes of joining bodies are based upon the 

 orce of adhesion. Glass plates may be rendered so 

 erfectly even by skilful workmen, that they cannot be 

 gain separated, if once in contact, without breaking. 

 >ut inmost cases a better contact is ensured by bringing 

 ome liquid or soft substances between the surfaces to 

 e joined ; after drying, the interposed substance is 

 early everywhere in contact with both surfaces, and 

 le separation cannot be effected without considerable 

 )rce. The processes of glueing, pasting, soldering, 

 tc., are such applications of the force of adhesion. 



Plates of glas s well adapted for experiments on adhesion may be 



spared with some little trouble and patience ; such plates will 



so be found to be very useful in the experiments on the pressure 



water upon the sides of vessels, and it is therefore well worth 



rile to devote some time to their preparation. Procure two 



cular discs of plate, or very flat window glass, 6 cm in dia- 



;3ter. They may be cut circular either by a pair of compasses 



rich have a small diamond fixed at one point, or with the help of 



stille. Cut a circular disc of paper of the required size, place the 



jtss upon it, and draw the outline of the circle with ink upon the 



j-s. When the ink is dry commence a crack from the edge of the 



] 'ie, and lead it along the circular line for some distance. Then 



l^in a new crack from another point of the edge, as shown in fig. 



H, and break away cautiously any pieces that can be removed. 



