78 Mr. J. Brown. [June 16, 



the same temperature as its chloride, it was first fused in the bend of 

 the tube A, and then its chloride fused on it in the shorter limb. 

 The melted metal was connected to the Thomson electrometer by a 

 wire of either copper or iron dipping in it. When this wire was of 

 copper, long contact with the fused metal seemed to dissolve it some- 

 what ; and a slight error may thereby be introduced in the cases of 

 tin, zinc, and cadmium ; but this is probably not large in any case. 

 For these fusible metals, a clay tobacco pipe was sometimes substi- 

 tuted for the tube A, the connecting wire passing through its stem. 

 When the metal in the small tube was of this easily fusible kind, it 

 was found best to add another asbestos plug, as at e, fig. 2, which 

 was permeated by the chloride below it at &, and supported the fused 

 metal g with its connecting wire /. 



While all mixture of the two chlorides was to be avoided as far as 

 possible, it was considered especially necessary to keep the chloride 

 of higher combining heat, which to save repetition we may call H, 

 from being permeated by that of lower combining heat L, for the 

 obvious reason that, when L mixed with H, its metal became at once 

 reduced on the metal in H, and the conditions were thereby altered. 

 If, for example, in the cell 



Copper | Cu 2 Cl a | ZnCl 2 | Zinc 



the copper chloride (L) diffuses into the zinc chloride (H), copper 

 becomes reduced on the zinc, and the electromotive force is lowered. 



To obviate or mitigate this difficulty, the following precautions 

 were taken : The V'tuhe ^ was ^ rs ^ se ^ a P> an d the chloride fused 

 in it. When the arrangement was such that chloride H was in the 

 small tube B, this tube was first held in the Bunsen flame till the 

 chloride fused and saturated the porous plug to the end or nearly so. 

 The tube B was then inserted in its place, any small mixture of H 

 with L not being considered of importance. If, on the other hand, 

 H was in A, and L in B, then B was simply heated till L was about 

 fusing point, and was immediately irserted in A, so that H had time 

 to soak up into the plug before L soaked down. 



The following was the usual plan of observing the electromotive 

 force : As soon as the electrolytes were in order, and the metals, 

 previously connected to the electrometer, were inserted, the deflexion 

 was noted immediately, and again after about five minutes. The 

 metals were then, if solid, taken out, cleaned, and re-inserted, and 

 the observations repeated. In these sets of observations the variation 

 from the mean of any given experiment was less with fused metals 

 than with solid metals, the latter being more liable to surface altera- 

 tions. The adopted mean of each set was derived from those 

 experiments in which the theoretical conditions were most strictly 

 fulfilled. 



