4 PHYSICS. 
solid ; when the cohesion and repulsion seem to be equal, the particles 
are less firmly united, are freely movable among one another, and the 
body is said to be liquid; when the repulsion is greater than the 
cohesion, the particles are still more freely movable than in liquids, and 
such bodies are called gases. 
Certain names have been applied to different states of cohesion: 
(1) Tenacity is the quality by which a body resists being torn asunder, 
and depends on the intensity of the cohesive force. An iron wire one- 
tenth of an inch in thickness will sustain a weight of 700 pounds. 
Fibrous substances, as silk and flax, possess great a The most 
tenacious of all substances is steel. 
(2) Malleability means the capability possessed by some meade as gold, 
silver, copper, &c. of being hammered out into thin plates. This poses 
depends on the union of ee and tenacity in the bodies possessing it ; 
being soft, their particles can be made to change their position with regard 
to one another; being tenacious, the particles will not readily separate. 
(3) Ductility, the capability possessed by a metal of being drawn out 
into wire, is of the same nature as Malleability, both depending on a 
certain degree of softness and great tenacity. These properties are not, 
however, identical, for the most malleable metals are not the most 
ductile. The most malleable metal is gold ; the most ductile, platinum. 
(4) Hardness——Both in malleable and in hard bodies, the force with 
which the particles stick together is very great. In a malleable body the 
particles can be made to change their position by sliding or rolling 
on one another without separating, while in a hard body they resist 
change of position, and, if forced, separate or break. The relative 
hardness of two bodies is ascertained by trying which of them will 
scratch the other. Thus glass will scratch gold, and even platinum. 
All precious stones are very hard. The diamond is the hardest substance 
known, and is used in cutting glass. , 
(5) Brittleness is closely allied to hardness, for most hard bodies are 
brittle. It would seem to be the opposite of Malleability, for on a very 
slight attempt to displace the position of the particles, the body flies in 
pieces. Glass has this property to a remarkable degree. 
The five properties enumerated above are due to certain states of the 
cohesion of the particles of a single solid body ; there is also an attraction 
between two different bodies, which makes them stick to each other by 
their surfaces. This attraction is called adhesion. 
5. Adhesion.—(1} Adhesion between two solids.—If two lead bullets be 
taken, and a piece be cut off each, leaving perfectly smooth surfaces, the 
bullets will stick together when these two surfaces are joined ; or if two 
pieces of glass be laid upon each other, they stick together with con- 
siderable force. (2) Adhesion between a solid and a liquid is seen 
