26 



HYDROSTATICS. 



and if the bores of the tubes diminish 

 in proportion as they are nearer the end 

 of the row, the liquid will stand in 

 them at different heights, so that its 

 heights in the different tubes will be in 

 the same curve line as the upper edge 

 of the liquid between the plates. 



The height to which water rises in 

 the tube, we have seen, is an inch 

 when the diameter of the tube is 

 l-50th of an inch, and it rises be- 

 tween the plates to the same height 

 when they are twice as near, or only 

 1-1 00th inch asunder. But this differ- 

 ence is what might be expected, upon 

 the supposition that the rise is owing to 

 the attraction of the glass ; for in the 

 tube there is glass all round, and it may 

 be supposed to be a small square, each 

 of the four sides acting on the water ; 

 whereas the plates have only two sides 

 acting, and therefore little more than 

 half the power. 



The action of the tubes or plates upon 

 liquids depends upon the nature of the 

 solid substances of which they are 

 made. If the glass is smeared with 

 grease so that water will not stick to it, 

 that liquid will not rise at all ; nor will it 

 rise between two cakes of wax or of 

 grease. So different liquids rise to dif- 

 ferent heights in the same tube ; but not 

 according to their specific gravity; for 

 oil of turpentine, which is one-seventh 

 lighter than water, only rises one-fourth 

 as high ; and aqueous* ammonia, which 

 in the table of specific gravities appears 

 to be about a tenth lighter than water, 

 and consequently heavier than oil of 

 turpentine, rises higher than water by 

 nearly a fifth, and consequently nearly 

 five times higher than oil of turpen- 

 tine. Spirit of wine, somewhat lighter 

 than oil of turpentine, rises nearly 

 twice as high. Mercury does not rise 

 at all, either between the plates or in the 

 tubes : on the contrary, it sinks con- 

 siderably lower than its level outside 

 the tube or plate. In a tube of 1 -5th 

 inch bore, it sinks l-5th of an inch ; and 

 in a tube of 1-1 Oth inch bore, it sinks 

 about 1-1 1th of an inch; so that the 

 sinking is n the same proportion to the 

 bore of the tubes in mercury as the rising 

 in watery fluids, being inversely as the 

 diameters ; and melted lead is found to 

 .sink in the same manner, and accord- 

 ing to the same rule. Again, it is ob- 

 servable that mercury, (and probably 

 melted lead,) which sinks in this way, 

 was always a round surface, and never 



stands up at the edge of the vessel con- 

 taining it, unless the vessel be made of 

 silver, or tin, or some other substance 

 with which mercury has a strong che- 

 mical affinity. Watery fluids sink in- 

 stead of rising in tubes, or between 

 plates, which are oiled or waxed. The 

 thickness of liquors seems to have no 

 influence on their capillary attraction, 

 nor has their heat ; for white varnish, 

 which is exceedingly thick and viscid, 

 rises nearly as high as spirit of wine, 

 and hot and cold water stand at the 

 same height. 



It is one consequence of capillary 

 attraction, that when light bodies, capa- 

 ble of being wetted by any liquid, float 

 upon it, and come very near each other, 

 they are drawn together as if they 

 attracted one another; but this is owing 

 to the fluid being raised on their sides, 

 and when very near it becomes raised 

 between them and attracts them. The 

 same thing happens when twj light bo- 

 dies, not capable of being wetted, float 

 very near, for they make the fluid sink 

 between them, and the bodies are 

 pressed together by the surrounding 

 fluid. But when one body is capable 

 of being wetted, and the' other not, 

 they are driven back one from the other, 

 because the fluid being raised by the 

 wetted body, on one side of the dry 

 ^ody, and sunk on the other, the dry 

 body is, as it were, on a slope, and 

 falls away from the other. 



Capillary attraction performs many 

 important offices in nature. Probably 

 the distribution of moisture in the 

 earth is regulated by it ; and there is 

 no doubt that the distribution of the 

 juices in plants depends principally 

 upon it. The rise of the sap and its 

 circulation is performed in the fine 

 capillary tubes of the wood and bark, 

 which are the arteries and veins of ve- 

 getables. Any one may perceive how 

 this process is performed, by twisting 

 together several threads of cotton or 

 worsted, and wetting them. If they 

 are then put in a glass of any co- 

 loured fluid, as red w r ine or ink, and 

 allowed to hang clown to the plate on 

 which the glass stands, the fluid will 

 soon be perceived to creep up, and 

 colour the whole of the threads, red or 

 black, as the case may be ; and in a 

 short time the whole contents of the 

 glass will come over into the plate. 

 Capillary tubes may in this manner 

 carry juices upwards, and distribute 



