PHYSICAL PEOCESSES IN CELLS. 39 



occurs in all eases, provided the heat to which the} - are subjected does 

 not cause them to undergo chemical change. If a body which has been 

 fused by heat has its temperature still further raised its molecules 

 become so much further separated that the force of cohesion is not 

 sufficient to keep them in contact, and the body then becomes vaporized. 



In liquids the force of cohesion is not great ; hence their molecules 

 are readily displaced and a mass of liquid assumes the shape of the vessel 

 which contains it ; in other words, the force of gravity overcomes the 

 force of cohesion. Cohesive force is, nevertheless, present in liquids, and 

 may be demonstrated by the difficulty with which a plate of glass placed 

 horizontally in contact with the surface of water is removed vertically 

 upward. This difficulty is due to the fact that the adhesion of the glass 

 to the water being greater than the cohesion of the water, the molecules 

 of water must be violently separated to permit of the removal of the 

 glass. 



The spheroidal form assumed by small masses of liquid, as in a drop 

 of dew, a globule of rnercury, is due to the working of the force of cohe- 

 sion of the molecules of the liquid. In a small drop of mercury placed 

 on a surface for which it has no adhesion, as wood or glass, the sum of 

 the mutual attractions of all its molecules being greater than the force 

 of gravity acting on them, the globule assumes the spherical form. If, 

 however, the drop of mercury is large, then the force of gravity increas- 

 ing with the mass of the body becomes greater than its cohesion and the 

 drop becomes flattened. 



The molecules of all liquids attracting each other, it is evident that 

 the molecules in the free surface of a liquid will be attracted by and will 

 attract those below them, but will exert or will be subjected to no exter- 

 nal attraction. At the surface of liquids, therefore, there is always an 

 inward attraction, which is called surface tension. Of course, in these 

 considerations external accidental pressures and attractions to which the 

 liquid may be subjected are neglected. 



The surface tension of liquids is well illustrated by blowing a soap- 

 bubble on the end of a glass tube ; as long as the other end of the tube 

 remains closed the elastic tension of the air in the bubble balances the sur- 

 face tension of the soap-film, but when the end of the glass tube is opened 

 the tension of the film leads to the contraction and final disappearance 

 of the bubble. So also insects can move on the surface of water without 

 sinking, for the water, not being able to wet their feet, forms a depression, 

 and the elastic reaction of the surface supports them. The case is simi- 

 lar when a sewing-needle is floated on water ; as the needle is coated 

 with a thin film of oil the water does not adhere to it, the surface becomes 

 depressed, and its increased tension serves to support the weight. Wash- 

 ing the needle first in alcohol, ether, or potash causes it at once to sink. 



