44 DISTURBING ACTION OF LEAKAGE. 



142. Before, however, proceeding to the detail of these experiments, it is necessary 

 to allude to certain disturbing circumstances which take place, arising from extraneous 

 mechanical action, and vitiating the result. One of the most prominent of these is due 

 to the general leakage which happens through the open pores of all tissues ; a leakage 

 which is to be distinguished from the proper capillary transit. If, for example, a bar- 

 rier of peritoneum be placed over the mouth of a vessel of water, under ordinary cir- 

 cumstances the escape of the water will be prevented; but if a pressure gradually in- 

 creasing be exerted on the water, it will rapidly ooze through every pore, and finally, 

 if the membrane stand the strain without rupture, will spirt through those of a large di- 

 ameter. This effect, to a greater or lesser extent, takes place wherever tissues have to 

 resist mechanical pressure ; the amount of disturbance arising from it depends mainly 

 on the diameter of the pores of the structure. 



143. In the experiment related in section 139, we have a well-marked instance of 

 this disturbance. It might be inferred from that experiment, that the force with which 

 water passed through a piece of peritoneum into alcohol was not greater than one at- 

 mosphere ; whereas, in truth, it was much more ; but, as soon as the pressure within 

 the vessel by the infiltration of water had amounted to about one atmosphere, the alco- 

 hol escaped from the vessel as rapidly as the water entered, by general leakage from 

 the whole surface of the membrane, and the gauge, therefore, gave no evidence of the 

 passage of the liquid. Nor are very porous structures alone liable to this accident; the 

 experiment of the old Florentine academicians shows, that even through the pores of 

 gold, one of the densest of the metals, fluids under a severe pressure will find their way, 

 as appeared when they attempted to compress water in a gbbe of that metal. 



144. This accidental passage through pores may be made visible to the eye by con- 

 densing about one atmosphere of air into a vessel whose mouth is closed by a sheet of 

 India-rubber, and then placing it in a jar filled with water ; small bubbles of air will be 

 seen escaping from every part of the India-rubber, and passing in great numbers through 

 the water. 



145. It has just been stated, that the force with which water passes through a mem- 

 brane into alcohol is much more than one atmosphere ; this may be proved by making 

 use of a barrier of a stouter fabric than the peritoneum here mentioned. A piece of 

 bladder being used in lieu of it, the gauge indicated, when the pressure was a maximum, 

 a force of 1'8 atmospheres ; but even this cannot be taken as the true value of the 

 force, for a certain period of time elapses, amounting, in this instance, to almost two 

 days, before the gauge reaches its highest point ; and when that is gained, the alcohol 

 has become considerably diluted, and agreeably to a law which will hereafter be pointed 

 out, the amount, of force rapidly diminishes as this takes effect ; for, as soon as the 

 composition of the fluids on both sides of the bladder is the same, provided the temper- 

 ature of both is alike, and no mechanical disturbance arises from unequal pressure, all 

 motion either way ceases, and this may happen long before the column of fluid in the 

 gauge has reached its highest point. 



146. The air gauge, however, at the best, is a very imperfect indicator of the force 

 with which gases or liquids mingle, for it will remain stationary, even when the passage 



