8 FERGUSON'S LECTURES. 



LKCT. strongly, as to require a force much greater than their 

 ^^^^ own weight to separate them. And this cannot be 



owing to the pressure of the air, for the same thing will 



hold in an exhausted receiver. 



5. If two polished plates of marble or brass be put 

 together, with a little oil between them, to fill up the 

 pores in their surfaces, and prevent the lodgement of 

 any air, they will cohere so strongly, even if suspended 

 in an exhausted receiver, that the weight of the lower 

 plate will not be able to separate it from the upper one. 

 In putting these plates together, the one should be 

 rubbed upon the other, as a joiner does two pieces of 

 wood when he glues them. 



6. If two pieces of cork, equal in weight, be put near 

 each other in a basin of water, they will move equally 

 fast toward each other with an accelerated notion, until 

 they meet : and then, if either of them be moved, it will 

 draw the other after it. If two corks of unequal weights 

 be placed near each other, they will approach with 

 accelerated velocities inversely proportioned to their 

 weights : that is, the lighter cork will move as much 

 faster than the heavier, as the heavier exceeds the 

 lighter in weight. This shews that the attraction of 

 each cork is in direct proportion to its weight, or quan- 

 tity of matter. 



This kind of attraction reaches but to a very small 

 distance ; for, if two drops of quicksilver be rolled in 

 dust, they will not run together, because the particles of 

 dust keep them out of the sphere of each other's 

 attraction. 7 



Note 7. This experiment may be shewn more perfectly, if the seeds 

 of lycopodium be employed ; as the fluid rings, which really produce 

 the effects described in the preceding paragraph are not in this case 

 allowed to operate. The seed already alluded to may be strewed 

 upon a table, and water poured over its surface, without wetting the 

 wood beneath. 



