ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



697 



reservoir. The construction of the latter, in which alone the battery 

 diflfers from that of Poggendorf, depends on the fact observed by Eadiguet 

 that when mercury containing traces of zinc is in contact with copper, 

 the current tends to transport the mercury over the whole surface of the 

 coiiper. It consists of a copper tube, coated with zinc, carrying a sort 

 of basket which holds the zinc in the form of small balls. Beneath the 

 basket is a porcelain dish which contains the amalgam, and is connected 

 by a copper rod to the two metal pieces forming the base of the basket. 



To start the battery an acid solution of sodium bichromate is poured 

 into the outer jar and pure water into the porous pot. The liquid in 

 the latter requires changing 

 every week, and that in the 

 outer jar every month. This 

 is easily effected, without 

 dismounting the battery, by 

 the use of the Eadiguet 

 siphon. The larger arm of 

 the siphon is connected by 

 tubing with a caoutchouc 

 ball, and incloses the nar- 

 rower tube which forms the 

 other branch. The lower 

 extremity of the larger arm 

 is narrowed, so that when 

 the ball is gently pressed, 

 the increase of pressure in 

 the tube forces the liquid 

 into the narrow branch and 

 the siphon functions. By 

 a strong quick pressure, on 

 the contrary, the liquid is 

 driven completely out of 

 the tube, and the action is 

 stopped at will. 



The EMF of the battery 

 is about two volts, and the 

 best arrangement for main- 

 taining a steady light for a 

 considerable time is to unite 

 for quantity two series of 

 three elements. 



The apparatus designed 

 by Prof. Engelmann (fig. 91) consists of a copper base M carrying the 

 rheostat E and lamp. The path of the current is seen from the figure, 

 connection between rheostat and lamp being made by a rod of copper. 

 The lamp can be adjusted in height by means of the two copper tubes 

 S sliding one within the other, and can be brought into any position by 

 means of the ball-joint. The rheostat consist of a cylinder of copper 

 insulated from the base by ebonite or serpentine, and containing a pile 

 of thin discs formed by a mixture of graphite and gelatin. By means 

 of the screw the discs can be more or less compressed together, and 

 thus the resistance regulated with great nicety. 



An improved form of the apparatus, constructed by IVI. Kagenaar, 



1889. ' 3 C 



