420 Prof. J. J. Thomson and Mr. H. F. Newall. [Dec. 17, 



^ inch in diameter, the liquid filtering slowly through a small filter 

 paper held by a wire ring in the upper end of the glass tube. The 

 tube and filter are fixed at one end of a light lever, which is counter- 

 poised at the other end and balanced carefully on a knife edge. The 

 lever was made of a narrow strip of brass foil, folded for the sake of 

 increased rigidity into a A -shape. On the lever between the knife 

 edge and the adjustable counterpoise a small piece of platinum is 

 fixed, and when the balance is set this is pressed lightly upwards 

 against a platinum point which is held in position by a brass arm. 

 The knife edge is connected with one terminal, the brass arm with 

 the other terminal of a circuit containing one or two Leclanche cells 

 and a Morse sounder, which we have adapted so as to serve as a 

 contact breaker in the spark circuit. The moving brass lever of the 

 Morse is prolonged by means of a light wooden arm, and the arm 

 carries a wire tipped with platinum and bent so as to dip into a 

 mercury cup. A current from three or four Grove cells is passed 

 along this wire, and the circuit is completed through a strong electro- 

 magnet. The mercury cup is placed conveniently so as to illuminate 

 the drop as it falls. 



In using the apparatus the following is the procedure : — Several 

 drops are passed into the filter paper, and the counterpoise is adjusted 

 so that contact is just made at the platinum points. Thus, the relay 

 circuit is complete, and as a consequence the lever of the Morse is 

 held down, and so the electromagnet circuit is made by the dipping 

 of the platinum tip into the mercury cup. The spring and counter- 

 poise of the relay and the mercury cup have been ''set" so as to 

 give the spark at break at abont the moment required. When the 

 drop detaches itself from the tube the equilibrium of the balance is 

 upset, the counterpoise sinks, and so breaks the relay circuit, which 

 consequently breaks the electromagnet circuit at the mercury surface, 

 and the spark so produced illuminates the drop at some point in its 

 descent. This point may be varied by altering the " setting " of the 

 relay and mercury cup. 



Various liquids were used for dropping : fluorescein dissolved in 

 weak ammonia gave very good results for two or three drops when 

 dropped into water, but it soon diffused through the column of water y 

 and the drops were then no longer clear, as the efficient rays had been 

 absorbed at the surface of the column. Milk gave excellent results ; 

 but even when considerably diluted it is disagreeable to work with on 

 account of the greasiness that accumulates in the dropping tube, and;; 

 of the cloudiness produced in the column. A weak solution of nitrate 

 of silver dropped into a column of weak sodic chloride solution was 

 found to do best, as very good rings are formed, and the precipitate 

 may be discharged by the addition of a few drops of ammonia. The 

 results figured below are those obtained in this way. 



