COHESION AND ADHESION. 11 



Expt. 8. To take Casts in Sulphur. Melt some sulphur in a 

 ladle, taking care that it does not take fire ; should it do so cover 

 the ladle with a card so as to exclude air, which will extinguish 

 the flame. When completely melted the heat must be withdrawn, 

 otherwise the sulphur will thicken again when over-heated, and 

 become too thick to pour properly. The melted sulphur may be 

 used to take casts just as the nearly-fluid plaster of Paris " slip " 

 in Expt. 3. Silver coins must not be thus treated, as they will be 

 blackened, and probably blacken the cast too ; other metals should 

 be slightly oiled to prevent the sulphur sticking to them ; if the 

 mould is of plaster, it should be previously moistened with water. 



Cohesion and Adhesion. 



The difference between a mass of solid stone and the same 

 substance reduced to powder by repeated blows with a hammer 

 and triturating the fragments in a mortar, or between a bar 

 of iron, and the same after being reduced to small fragments 

 by a file, is that in the one case the component particles are 

 so situated with reference to one another that there is great 

 cohesion between them, and consequently the solid mass pos- 

 sesses great power of resistance to forces tending to break it 

 asunder ; whilst in the other case the granules, dust, or filings do 

 not cohere together at all. In some cases subjecting a powdered 

 substance to powerful pressure will cause its particles to cohere 

 together to some considerable extent ; well polished perfectly flat 

 glass plates laid one on top of another will sometimes cohere 

 (without any cement) so perfectly that it is almost impossible to 

 separate them without breaking the glass. Freshly-cut clean 

 surfaces of soft solids will often cohere perfectly on pressing 

 together; thus two bright surfaces of freshly-cut lead or india- 

 rubber will often stick together with great force in this way. 

 Liquids exhibit cohesion as well as solids; one result of which 

 is the rounded shape assumed by drops of dew in the morning on 

 plants, or by drops of water scattered over a dusty surface. 



Adhesion is the term applied to the tendency of dissimilar 

 kinds of matter to stick together, cohesion being precisely the 

 same kind of thing with substances when the constituent particles 

 are of the same kind of matter. When the hand is dipped into 

 water it becomes wet, the water adhering to the skin; a well- 

 greased finger, however, may be dipped into water without being 

 wetted on account of the smallness of the adhesion between the 

 greasy film on the skin and the water. The use of cements, 

 paste, glue, solders, and such like uniting materials is due to the 



