232 SCIENTIFIC AMUSEMENTS. 



in this more general kind of action the gas is only attracted to the 

 surface of the solid, and does not penetrate into the interior except 

 by means of cavities or pores, which are realty only prolongations 

 of or convolutions in the outer surface. Thus, if a solid piece of 

 glass or porcelain be heated pretty hot, quickly cooled, and weighed 

 on a sufficiently delicate balance (fig. 49), the moment it has cooled 

 sufficiently to prevent currents of warm air being set up from its 

 surface and so disturbing the action of the weighing instrument, 

 it will be found to weigh perceptibly less than after standing 

 awhile, because during this standing air is attracted to the surface 

 of the object and adheres pertinaciously to it, causing it to increase 

 in weight just as though it had been slightly wetted with water. 

 On heating it again this air will be to a considerable extent driven 

 off", so that, on weighing again after cooling, the weight will be 

 much the same as at first, and perceptibly less than at the second 

 weighing after standing awhile. This condensation of air on the 

 surface of glass and other vessels introduces considerable difficulties 

 in various delicate chemical operations, where substances require 

 to be weighed in containing vessels with a high degree of accuracy ; 

 where the surface of the glass is made very large in proportion to 

 its weight (as by drawing out the glass into fine thread or " spun 

 glass " whilst molten by a blowpipe), the bulk of gas thus attracted 

 to the surface is very considerable indeed ; and similarly with 

 other solids. The effect of the film of air condensed upon the 

 surface of solids is in some cases to prevent their coming into 

 true contact with liquids in which they are immersed, so that no 

 "wetting" ensues, even though under other circumstances the 

 fluid would wet the solid freely. 



Expt. 279. To make Solid Steel float on Water. A thin 

 bright sewing needle may often be carefully deposited on the sur- 

 face of a basin of water, and will float thereon without sinking, 

 because a film of air adheres to it, preventing the water from touch- 

 ing it, and buoying it up as a cork would do, but more effectively. 

 If you attentively examine the liquid you will observe that the 

 needle lies in a depression, which is considerably greater in magni- 

 tude than the needle itself, the air filling this depression being 

 virtually attached to the needle for the time being, and thus enabling 

 it to displace a much larger column of water than its own bulk, 

 and therefore to float. This action is closely connected with 

 certain phenomena called capillarity and surface tension, 

 referred to hereafter (Chapter XVII). 



In filling barometers the adhesion of air to the glass is a source 

 of error, as this air is apt to creep upwards gradually into the 

 "Torricellian vacuum" at the top of the instrument, and thus spoil 



