1876.1 



on the Leaf of Dionsea muscipula. 



419 



with a cul-de-sac of thick india-rubber (not shown in the diagram), the 

 cavity of which, as well as that of the tube itself, is, with the exception 

 of a small bubble of air, filled with mercury. The cul-de-sac can be com- 

 pressed by a screw-clamp, so that the mercury in A can be subjected to 

 the pressure required in order to force it through the capillary, or to any 

 less pressure. The tube B also contains mercury at its closed end : the 

 remainder of its cavity is occupied by dilute sulphuric acid, into which the 

 end of the capillary y is plunged. Each of the two tubes A and B has a 

 platinum wire fused into it, by which the two masses of mercury can be 

 severally brought into connexion with any two surfaces of which it may 

 be desired to compare the electrical condition. 



The instrument is prepared for use by first increasing the pressure in 

 A until mercury escapes from y, and then diminishing it, until it stands 

 at a point previously fixed upon (which may be called the zero point 

 care being taken that the wires a and (i are in contact ivith each other. If, 

 instead of touching each other, any electromotive arrangement or struc- 

 ture is interposed between them, as (<?. g.) a muscle-cylinder, the mercurial 

 meniscus in the capillary alters its position, retreating from y if the sur- 

 face with which it is connected is negative to the other, and vice versa. 

 The difference of tension thus indicated may be measured, either by 

 increasing or diminishing the pressure in A, so as to bring back the 

 meniscus to its zero position, and measuring the change of pressure 

 required for the purpose (" compensation pressure "), or by measuriug 

 the distance of the point at which the mercurial column in the capillary 

 stands when the terminals are in contact with each other from that at 

 which it stands when the electromotive structure is interposed. For 

 physiological purposes the latter plan is the most convenient. In order 

 to carry it out, the capillary must be mounted on the stage of a suitable 

 microscope, and furnished with micrometrical arrangements capable of 

 measuring accurately to a hundredth of a millimeter. 



The electrical values to be assigned to the measurements so obtained 

 must be learned in respect of each instrument by a preliminary process 

 of empirical graduation. 



Each capillary used as an electrometer must be graduated. The 

 graduation is effected by Poggendorff's method of compensation, with 

 the aid of Mr. Latimer Clarke's potentiometer. As a standard cell 

 we have used a silver-chloride element, kindly given us for the pur- 

 pose by Mr, De La E-ue. The coil of our potentiometer consists of 

 60 turns of platinum wire of 0*26 millim. in thickness. The ope- 

 ration of graduating is rapidly performed ; consequently it can be 

 repeated frequently to ensure accuracy. The reasons why for our 

 purpose we preferred the electrometer to the galvanometer in the 

 present investigation are easily made clear. In all investigations 

 relating to the electrical phenomena of plants and animals the object in 

 view is to ascertain in what way any changes in the electrical condition 



