COHESION. 



i. e. by a relative rest among themselves. 

 For myself; it rather appears to me that 

 the particles of bodies cohere by an attrac- 

 tive force, whereby they tend mutually to- 

 ward each other; which force, in the 

 very point of contact, is very great ; at 

 little distances is less ; and at farther dis- 

 tances is quite insensible." 



But, whatever the cause of cohesion 

 may be, its effects are evident and cer- 

 tain. The different degrees of it consti- 

 tute bodies of different forms and proper- 

 ties. Thus, Newton observes, the parti- 

 cles of fluids which do not cohere too 

 strongly, and are small enough to render 

 them susceptible of those agitations which 

 keep liquors in a fluid state, are most 

 easily separated and rarefied into vapour, 

 and make what the chemists call volatile 

 bodies ; being 1 rarefied with an easy heat, 

 and again condensed with a moderate 

 cold. Those that have grosser particles, 

 and so are less susceptible of agitation, 

 or cohere by a stronger attraction, are 

 not separable without a greater degree of 

 heat ; and some of them not without de- 

 composition. 



Modern chemists have agreed to con- 

 sider the attraction of cohesion as the in- 

 strument of aggregation, or the union of 

 similar compounds, and are careful not 

 to confound it with the elective attrac- 

 tions, though there may, in strictness, be 

 no difference between them. See CHE- 

 MISTRY. 



This kind of attraction is evinced by 

 a variety of familiar experiments ; as, by 

 the union of two contiguous drops of 

 mercury ; by the mutual approach of two 

 pieces of cork floating near each other in 

 a basin of water ; by the adhesion of two 

 leaden balls, whose surfaces are scraped 

 and joined together with a gentle twist, 

 which is so considerable, that if the sur- 

 faces are about a quarter of an inch in 

 diameter, they will not be separated by 

 a weight of WOlb ; by the ascent of oil 

 or water between two glass planes, so as 

 to form the hyperbolic curve, when they 

 are made to touch on one side, and kept 

 separate at a small distance on the other ; 

 by the depression of mercury, and by the 

 rise of water in capillary tubes, and on 

 the sides of glass vessels ; also in sugar, 

 sponge, and all porous substances. And 

 where this cohesive attraction ends, a 

 power of repulsion begins. 



It is uncertain in what proportion this 

 force decreases as the distance increases. 

 Desaguliers conjectures, from some phe- 

 nomena, that it decreases as the biqua- 

 dratic or 4th power of the distance, so 



that at twice the distance it acts 16 timer 

 more weakly, &c. 



To determine the force of cohesion, 

 in a variety of different substances, many 

 experiments have been made, and parti- 

 cularly by professor Muschenbroek. The 

 adhesion of polished planes, about two 

 inches in diameter, heated in boiling 

 water, and smeared with grease, requi- 

 red the following weights to separate 

 them. 



Cold Create. Hot Grease . 



Planes of Glass . 

 Brass . 

 Copper 

 Marble . 

 Silver . 

 Iron. 



Ib. 

 130 

 ISO 

 200 

 225 

 150 

 300 



Ib. 

 300 

 800 

 850 

 600 

 250 

 950 



But when the brass planes were made 

 to adhere by other sorts of matter, thr 

 results were as in the following table : 



oz. 

 With Water .... 12 



Oil. ..... 18 



Venice Turpentine 24 

 Tallow Candle . . 800 

 Rosin ..... 850 



Pitch .... 1400 



In estimating the absolute cohesion of 

 solid pieces of bodies, he applied weights, 

 to separate them according to their 

 length : his pieces of wood were long 

 square parallelopipedons, each side of 

 which was 26 of an inch, and they were 

 drawn asunder by the folio wing weights ; 



Fir ........ 600 



Elm ....... 950 



Alder ....... 1000 



Linden tree ..... 1000 



Oak ....... 1150 



Beech ..... . 1250 



Ash ...... , 1250 



He tried also wires of metal, l-10th of 

 a Rhinland inch in diameter : the metal? 

 and weights are as follow : 



Ib. 

 Of Lead ...... 29J 



Tin ...... 40 



Copper ..... 299 



Yellow brass . . .360 

 Silver ...... 370 



Iron ...... 450 



Gold ...... 500 



