CONSTITUTION OF BODIES. 



body said to be ^J; when it is great the body is said to be touyh. A 

 body which becomes 8awed or broken before it can be permanently defoniu-,1 

 w called brittle. When the force required is great the body is said to be 



The stiffness of a body is measured by the force required to product a 

 amount of deformation. 



It* strength is measured by the force required to break or crush it. 



We may conceive a solid body to approximate to the condition of a fluid 

 in several different ways. 



If we knead fine clay with water, the more water we add the softer does 

 the mixture become till at last we have water with particles of clay slowly 

 subsiding through it This is an instance of a mechanical mixture the rn- 

 stttoents of which separate of themselves. But if we mix bees-wax with oil, 

 or rosin with turpentine, we may form permanent mixtures of all degrees of 

 softness, and so pass from the solid to the fluid state through all degrees of 

 viscosity. 



We may also begin with an elastic and somewhat brittle substance like 

 palatine, and add more and more water till we form a very weak jelly which 

 opposes a very feeble resistance to the motion of a solid body, such as a spoon, 

 through it. But even such a weak jelly may not be a true fluid, for it mav 

 be able to withstand a very small force, such as the weight of a small mote. 

 If a small mote or seed is enclosed in the jelly, and if its specific gravity is 

 different from that of the jelly, it will tend to rise to the top or sink to the 

 bottom. If it does not do so we conclude that the jelly is not a fluid but 

 a solid body, very weak, indeed, but able to sustain the force with which the 

 mote tends to move. 



It appears, therefore, that the passage from the solid to the fluid state 

 may be conceived to take place by the diminution without limit either of the 

 coefficient of rigidity, or of the ultimate strength against rupture, as well as 

 by the diminution of the viscosity. But whereas the body is not a true fluid 

 till the ultimate strength, or the coefficient of rigidity, is reduced to zero, it 

 in not a true solid as long as the viscosity is not infinite. 



Solids, however, which are not viscous in the sense of being capul>li 

 iui unlimited amount of change of form, are yet subject to alterations depend- 

 ing on the time during which stress has acted on them. In other words, th- 

 at any given instant depends, not only on the strain at that instant. 



