ELECTROMOTIVE FORCES IX THE VOLTAIC CELL. 



529 



metal 

 liich is 



nc, and 

 fcncc of 



in volts, 



the air, 



s cliavge 

 (i.e., the 



! eucrcjy) 

 eusioii is 



with air, 

 in regard 

 electrical. 



al,;i=Aa'?, 



jstimate of 



reasoning 

 It ia true 

 , wV 

 unless —- 



)n, copper, 

 the case; 

 one hand, 

 er platinum 

 and sodium 

 linness was 



^e of atoms 



Thomson 



KG of atoms 



[states these 



Letre, hrass 



loms arc s'J 



JO regarded 



■elf 280' by 

 llrchness. 



The theory of gaseous collision, combined with the density of liquids, 

 suggests a range lying between 7 x 10"" and 2 x 10-^. 



The dispersion of light seems to require atomic dimensions to Ho 

 between 10-^ and 1C^'\ 



The final estimate made by Sir Wm. Thomson is something between 

 2 xlO"^ on the one side, and 10-" on the other. But if the reasoning in 

 the present paper be admitted as correct, it wonld seem possible to reduce 

 this range of uncertainty and to make an even more precise estimate. 



I \\ 



On the Archcean Rocks of Great Britain. By Professor T. (I. Bonney, 

 D.Sc, LL.D., F.R.S., Pres. G.S., Fellow of St. John's College, 

 Cambridge. 



[A communication ordered by the General Committee to be printed in extenso 



among the Kcports.] 



Two methods of dealing with this subject, on which I have been asked 

 by the Organising Committee of Section C to prepare a paper, naturally 

 suggest themselves. The one is to treat it liistorically, by giving in 

 chronological order a precif of the papers or books in which i*eference has 

 been made to the Archivan rocks of Britain ; the other to describe, as 

 accurately as is possible in a limited space, the petrology of the several 

 districts, stating brief! j" at the same time the reasons which have led the 

 writer, in common with many other geologists, to consider their rocks as 

 more ancient than the Cambrian period. 



The former method has doubtless many advantages, and would be the 

 fit one, had I been called upon to ' report ' on the Archroan question ; 

 but it does not commend itself to me, in this particular case, as the one 

 most likely to be helpful to tliose who arc more especially engaged in 

 pctrological studies. Speaking for myself, I always tind it, in the case of 

 a district not yet visited, more useful to be informed as exactly as 

 may be what are the mineral and physical characters of its rocka, and 

 what their stratigraphical relations, than what opinions have been enter- 

 ta'aed as to their antiquity. For this purpose it is not enough to have 

 thcin named, unless the grounds of the writer's nomenclature are given. 

 There has hitherto been so much latitude assumed in the use of such terms 

 as ' schist ' and ' slate,' not to mention others, that they become of little 

 value unless what we may term the ' personal equation ' of the writer 

 be known. Accoi'dingly, in this paper, I shall endeavour to give as exactly 

 as is possible, without entering into minute details, the mineral characters 

 of the Archccan rocks in each district noticed, and the relations in which 

 they stand to those of ascertained geologic age. I may add that 

 throughout I shall use the torni ' schist ' to denote a more or less 

 foliated rock — that is, one in which presumably great mineral changes have 

 taken place since its materials were first deposited, so that, if they were 

 originally clastic, few, if any, traces of the constituent grains can be recog- 

 nised ; ' schistose,' to denote that a rock looks like a schist, but in this 

 case I imply no more than an external resemblance. By thi^ term meta- 

 riorphic rocks I mean those which, like schists, have undergone great 

 mineral changes — not rocks which, like many which are schistose, have 

 been in reality but slightly altered, whose changes have been only micro 



ill 



,11 



'MM 



mincralogical. The word 'slate 

 1884. 



is never applied to a foliated rock, 



