32 Mr. Martyn Roberts on the Analogy between the 



and its amount is the weight of the fluid multiplied by its ve- 

 locity. It will be found that the blood has but little momen- 

 tum, for its motion is slow, and that the heart possesses more 

 than sufficient power to overcome all the resistance offered 

 by the tortuosities of the vascular system. 



5. The adhesion of a fluid to the sides of the tube through 

 which it passes is a source of resistance that has been much 

 overlooked, but the retardation it would produce in the cir- 

 culation of the blood would be very considerable were it not 

 compensated or destroyed, especially in the capillary vessels, 

 which are of so small a diameter that their parietes touch 

 every corpuscle of blood that passes them ; though some few 

 of the capillaries are large enough to admit two or three 

 globules abreast, many can contain but one red particle of 

 blood in their diameter, while others are so minute as to ex- 

 clude the red corpuscles, admitting only the fine lymph. 



G. The adhesion of fluids to solids is so great, that if every 

 particle of a fluid be in contact with the sides of its channel, 

 a very considerable force will be necessary to overcome the 

 resistance ottered to its passage; and this adhesion can be 

 proved in the following manner: — Suspend the solid we wish 

 tried (say a bit of wood) to one end of a fine balance; to the 

 other end add its exact counterbalance in air, then place be- 

 neath the wood a vessel containing the liquid, and let the solid 

 rest upon the fluid surface ; add weights to the counterbalance 

 until the wood is raised from the liquid, and these weights will 

 give the amount of adhesion. Fig. 1 shows the arrange- 

 ment. The following is a list of the force of adhesion be- 

 tween a few solids and liquids: — A square inch of wood re- 

 quired 50 grains to detach it from the surface of water; a 

 square inch of gold 446 grains from a surface of mercury ; 

 silver, 429 grains; tin, 418 grains; lead, 397 grains; glass, 

 194 grains. 



7. Finding this adhesion to be so great, I concluded it to 

 be the main cause of the resistance offered by capillary tubes 

 to the flow of liquid through them, and that if the adhesion 

 could be destroyed, it would run through such tubes in direct 

 proportion to their diameters ; for it is well known that in or- 

 dinary cases (where, as I conclude, the adhesion exists) a 

 liquid will not flow through fine tubes in the ratio of their 

 diameter, but in a much less quantity than would be propor- 

 tionate to larger tubes; indeed, when the channel is capillary, 

 water is either completely arrested or runs in drops at long 

 intervals. 



8. Electricity has the power of destroying this adhesion 

 between liquids and solids, for bodies in a like state of electri- 



