

OM8TITI-TION OF 



the suUrtanoe is undoubtedly in the liquid state, f..r if we now diminish 

 pnwurr to sjsjlimlllf If" than 40 atmospheres the substance will exhibit 

 ordinary distinction between the liquid and the gaseous state, that is t<. 

 mr, part of it will evaporate, leaving the rest at the bottom of the vessel, 

 ith a dUtinct surface of separation between the gaseous and the liquid parts. 



The MUMUgn of a substance between the liquid and the solid state takes place 

 with Tarioua degrees of abruptness. Some substances, such as some of the more 

 crystalline metals, sesm to pass from a completely fluid to a completely solid 

 lit* very suddMilv. In some cases the melted matter appears to 'become 

 thicker before it solidifies, but this may arise from a multitude of solid crystals 

 being formed in the still liquid mass, so that the consistency of the mass be- 

 comes like that of a mixture of sand and water, till the melted matter in 

 which the crystals are swimming becomes all solid. 



There are other substances, most of them colloidal, such that when the 

 ! i it- 1 tod substance cools it becomes more and more viscous, passing into tlu- 

 olid state with hardly any discontinuity. This is the case with pitch. 



The theory of the consistency of solid bodies will be discussed in the article 

 -rii-iTY, but the manner in which a solid behaves when acted on by 

 furnishes us with a system of names of different degrees and kinds uf 



A fluid, as we have seen, can support a stress only when it is uniform in 

 itll directions, that is to say, when it is of the nature of a hydrostatic pressure. 



There are a great many substances which so far correspond to this defini- 

 te n of (i fluid that they cannot remain in permanent equilibrium if the stress 

 within them is not uniform in all directions. 



In all existing fluids, however, when their motion is such that the shape 

 of any small portion is continually changing, the internal stress is not uniform 

 in all directions, but is of such a kind as to tend to check the relative motion 

 of the part* of the fluid. 



This capacity of having inequality of stress called into play by inequality 

 !' motion is called viscosity. All real fluids are viscous, from treacle and tar 

 to water and ether and air and hydrogen. 



When the viscosity is very small the fluid is said to be mobile, like water 

 and ether. 



When the viscosity is so great that a considerable inequality of str 

 though it produces a continuously increasing displacement, produces it so slowly 



