184 



HEAT. 



the boiling-point, where the normal vapour pressure is atmospheric, so 

 that the excess may suffice to sustain the weight of the drop and we need 

 not think of it as held up by any outside mechanism. If the pressure 

 does not suffice, the drop comes lower down, nearer the source of heat, 

 the superheating is greater, while the escape round the edges is less, and 

 the pressure rises till the drop is sustained. 



A curious case of the spheroidal state may often be noticed when a 

 stream of water falls on a water surface. Of the drops which splash up 



and fall down again many will be seen 

 to remain some little time as drops 

 without coalescing with the general 

 body of the liquid. The effect often 

 occurs when, in rowing on still water, 

 the oars are held steady, and the drops 

 are allowed to fall on the hitherto un- 

 disturbed surface. And it is beautifully 

 illustrated when a fiddle bow is drawn 

 across the edge of a round glass vessel 



containing methylated spirit. As soon as the vessel vibrates regularly 

 a shower of drops is thrown by the vibrating segments of the glass 

 towards the centre, where they remain on the surface for a short time, 

 forming a most exquisite pattern. 



But though by sudden stretch of surface, and by evaporation in passage 

 through the air, the drops are doubtless colder than the general body of 

 the liquid, the temperature difference can hardly be enough to account for 

 the effect, which in this case is probably due, in some way not yet explained, 

 to surface tension. 



FIG. 107. 



