LECTURES. 203 



(61.) Cohesion of solids. — Two leaden balls made to cohere with a 

 force of 40 lbs. to square inch of surface of contact. Two glass 

 plates shown to cohere with great force — also two plates of marble. 



(G2.) The relative cohesion of bodies is called the tenacity, and this 

 is determined by the weight required to pull apart a bar of the sub- 

 stance an inch square. 



This weight is sometimes called the limit of cohesion and a know- 

 ledge of it is of great importance in the arts. 



Barlow's table of the cohesion of the principal substances used in 

 the art of construction. 



Cast steel 134,256 lbs. 



Swedish malleable iron 72,064 



Good American do 60,000 



English do 55,872 



Cast iron 19,096 



Cast copper 19,072 



Yellow brass , 17,958 



Cast tin 4,736 



Castlead 1,824 



Teak 12,915 lbs. 



Oak - 11,880 



Sycamore. 9,630 



Beech 12.225 



Ash 14, 130 



Elm... 9,750 



Memel fir 9,540 



Christiana deal 12,346 



Larch 12,240 



Considerable uncertainty in reference to tenacity — much smaller 

 force required to produce rupture, if time be allowed for the action. 

 Explanation of this. 



(63.) The tenacity and density of surface of metals are increased by 

 drawing the masses into wire. The cohesion of gold, silver, and 

 brass more than doubled by this process. \_Bohison.'] The surface in 

 this case appears to receive a fibrous texture. If the outside be re- 

 moved by acid the tenacity is materially lessened. Same effect pro- 

 duced by annealing [heating and gradually cooling] the wire. 



(64.) The mixture of some metals is more tenacious than the metals 

 themselves. Brass is stronger than its components, copper and zinc. 

 A small addition of zinc to tin almost doubles its strength. In these 

 cases heterogeneous attraction is stronger than homoi:eneou8. 



(65.) The tenacity of many substances is greater in some directions 

 than in others. Examples, crystals, wood, &c. 



The tenacity of bodies is effected by heat ; sometimes increased, 

 sometimes diminished. Iron at first stronger then weaker. 



The effect of a small degree of heat on the cohesion of two leaden 

 balls shown by experiment. By the application of a greater degree of 

 heat, the metal may be changed from a solid to a vapor. 



(66.) Cohesion of liquids . — The relative intensity of cohesive force of 

 liquids may be measured by suspending a plate, which can be wet by 

 the liquid, to the arm of a balance, and attaching weights to the 

 other arm until separation takes place. Dividing the weight thus 

 found, by the number of square inches in the plate, the quotient will 

 give the cohesive force for one square inch. 



The cohesion of water for water, shown by the force required to sep- 

 arate a disc of wood, ilupture between water and water. Attraction 

 of water for wood greater than that of water for water. 



In the same manner we can find the relative cohesion of different 

 liquids. 52 grains to the square inch required for the separation of 



