SiGERSON — Buoyancy of Bodies Denser than Water. 23 



of water attaches with an adhesive force greater than the force of 

 cohesion which keeps the molecules of water together ; for, if we raise 

 such a body out of the fluid there is rupture of this force, as a film of 

 water will still adhere strongly to the removed body. The adhesion 

 of air to an aerated, air- wetted, or (as commonly said) dry body is 

 likewise strong, in many cases, though not so open to remark. To a 

 lai'ge number of bodies, which come easily under our notice, water 

 adheres more vigorously than air; they are readily water- wetted. 

 But there are also some to which the air appears to adhere with greater 

 tenacitj". 



When an aerated body is placed on the surface of water, its atmo- 

 sphere, that is to say, the film of air which" surrounds it, tends to 

 increase its volume without increasing its weight. Being much lighter 

 than water, the adhering air will, therefore, co-operate with other 

 causes to prevent the body from sinking and to keep it buoyant, as long 

 as the force of air-adhesion endures. 



In order to demonstrate that air-adhesion supplies a force potent, 

 enough to act in tlie manner alleged, it is only necessary to take some 

 small bodies, whose density in relation to size shall not be very great, 

 but amply sufficient to cause sinking when water-wetted. In experi- 

 menting, I have found it most convenient to use seeds, taking different 

 kinds and sizes, generally round in form, such as the seeds of Everlast- 

 ing Pea {Lathyrtis latifolius), of Sweet Pea {Lathyrus odoratus), of 

 mustard, turnips, &c. If water- wetted, any of these objects will sink 

 at once, but when placed on the surface, dry (or aerated), they float. 

 There they illustrate all the phenomena of "capillarity" and "attrac- 

 tion" known in such cases. 



Whilst the under surface and sides have become water- wetted, there 

 is a dry patch above — in other words, to a portion of the upper surface 

 the air still adheres. A comparatively smart blow may be given here, 

 without causing such a body to sink — it will go down a little and re- 

 bound ; but if it be lightly touched (say a turnip-seed by a flat-ended 

 pencil), so that the water cover it, it sinks at once. That is to say — 

 the air-adhesion has been broken, the complementary cause of its 

 buoyancy. 



If we now take some dry or aerated seeds and drop them into the 

 water from various distances, say about a foot high, the force of the 

 air-adhesion becomes strikingly and beautifully manifested. It will 

 be immediately remarked that numbers of the seeds as they sink in the 

 water have small globes of air adhering to them. We may have the 

 following cases occurring: — ■ 



1. The body, falling from a height, may retain a globule of air ad- 

 hering to it, insufficient to counteract its own weight and the water- 

 pressure. It remains, therefore, at the bottom, and with it remains 

 the air-globule. The force of adhesion here manifested is stronger than 

 that which solicits the air to ascend to the surface, 



2. The body may carry down with it a globule sufficiently large to 

 raise it from the bottom, assisted perhaps by the rebound, if the shock 

 be not strong enough to sever the adhesion and set the globule free. 



