2o PHYSIOLOGICAL PHYSICS. [Chap, n. 



solution ; and (2) the deposition of the hydrogen 

 film is prevented by the recomposition of the liberated 

 H to form sulphuric acid. The Daniell is, by these 

 means, one of the most constant of elements. When 

 in good condition, the element may be worked for 

 hours without producing any amount of variation 

 of current. It is, therefore, specially valuable for 

 physiological purposes, where comparative experiments 

 are being made, and a uniform strength of current is 

 necessary. Usually, instead of having an outer glass 

 vessel, a vessel of copper is taken, which contains 

 the copper solution, and is provided with a copper shelf. 

 All that is further necessary is the porous cell, as an 

 inner compartment, with its cylinder of zinc and aci4. 



Copper is + pole, zinc - . 



A new form of Daniell, called the "gravity 

 element," depends on the difference in the density of 

 the two fluids for keeping them separate. An earthen- 

 ware vessel is taken, and in the bottom is laid a disc 

 of copper, on which is poured a saturated solution 

 of copper sulphate. Suspended by catches from the 

 top of the vessel is a sort of grating of zinc, which is 

 covered bv a solution of zinc sulphate. In the jar 

 there are thus two layers of fluid, one the layer of 

 copper solution at the bottom, and above it the layer 

 of zinc solution, and the difference in the density 

 prevents them mixing. The zinc grating is at the 

 surface of the uppermost fluid, and it has in its centre 

 a small opening through which crystals of copper 

 sulphate can be dropped to maintain the strength 

 of the lower stratum. One pole ( ) comes off from 

 the zinc ; the + pole is an insulated wire, which 

 passes through the liquid and is soldered to the copper 

 plate. All that is required to maintain the element 

 is occasionally to drop a few crystals of copper into 

 the solution, and to pour a little water on to the 



