NA TURE 



{May 2,, 1883 





CHEMICAL NOTES 



That the statement of the "law of isomorphism" given by 

 Mitscherlich is not applicable to all cases of isomorphous salts 

 has been recognised for some time. M. Klein has recently 

 described certain pairs of salts which crystallise in identical 

 forms, but are not of similar chemical composition ; thus tungsto- 

 boric acid, 9WO., . B 2 0, . 2H 2 G\ is isomorphous with silica- 

 tungstic acid, 12WO3 . Si0 2 . 4H 2 0. M. Klein proposes to 

 stale the law of isomorphism in the following terms : — " Iso- 

 morphous bodies have either similar chemical composition, or 

 exhibit only small differences in percentage composition ; they 

 contain either a common group of elements, or groups of ele- 

 ments of identical chemical formation which form by far the 

 greater part of their weight " (Compt. Rend, xcv. 781). 



The rare metal thorium has been obtained in some quantity 

 and in a pure -tate by Nils^n. The properties of this metal are 

 described in Compt. Rend. xcv. 727 et se</. : the sp. gr. is 11, 

 an t the atomic weight 232"36. 



F. M. Raoult (Compt. Rend. xcv. 1030) has studied the re- 

 duction of freezing- point of a liquid cau-ed by the solution in it 

 of a solid sub-tance. He concludes that a molecule of any 

 compound dissolved in 100 molecules of any liquid of a different 

 nature lower* the freezing-point of the liquid by a nearly 

 con-tant amount (about o°o2). This law, he asserts, is general 

 if it is admitted thit physical molecules maybe composed of 

 two, and in some few ca es of three chemical molecules. 



Wroblkwski (whose experiments have been already referred 

 to in these notes) states (Ann. Phys. Chan. [2], xix. 103) that if 

 a little water is introduced into a tube containing C0 2 , t' e 

 whole cooled to 0°, the pressure increased till the C0 2 liquefies, 

 and then suddenly released, care being taken lhat the pressure 

 does not fall below I2'3 atmospheres, a thin opaque solid forms 

 on the surface of the water, which solid is a definite hydrate of 

 carbon dioxide. Further experiments are detailed, showing 

 that the probable formula of this hydrate is CO s 8H 2 0. 



M. Spring continues his investigation on the influence of 

 great pressure on chemical action (Berichte, xvi. 324). He has 

 succeeded in preparing definite arsenides of zinc, lead, tin, 

 cadmium, copper, and silver. 



A NEW method for preparing the paraffins (C„H 2 „ +2 ) has 

 been found by Herr Kohnleio, a student in I'rof. Lothar 

 Meyer's laboratory at Tubingen ; the method consists in heating 

 together pure dry aluminium chloride and the normal iodide of 

 the paraffin radicle require I ; e.g. A1CI., and C 3 H 7 I yield pure 

 C 3 H 8 ; AICI3 and C 2 H 5 I yield pure C 2 H 6 , &c. 



After having published his important work on the etherisa- 

 tion of alcohol-, Prof. Menshutkin now publishes in the Journal 

 of the Russian Chemical Society a new paper on the methods of 

 qualitative determination of aniline and analogous bases which 

 have no alkaline reaction, as well as of triethylamine and 

 similar base-, and of ammonia. All the e meibods are a 

 generalisation of the method of alkalimetry, and the discovery of 

 them has afforded the author the po sibility of studying the 

 classic reaction of the permutation of bases in solutions of their 

 neutral silts. This last is the subject of his first paper. The 

 reaction being made under the most simple unvarying physical 

 conditions, M. Menshutkin begins with the study of complete 

 permutations, and shows that the theory of BertlWIet as to the 

 influence of the chemical mass is not true with regard to aniline, 

 which is c impletely substituted in salts by bases whose tempera- 

 ture of combination with hydrochloric acid is greater than for 

 aniline ; the same is true with regard to triethylami e, which is 

 also substituted completely, notwithstanding the increase of its 

 chemical mass, and to ammonia. These researches have led 

 the author to a new method of titration hy mean; of the alco 

 holate of barium, and to a means of tudying the formation and 

 dissociation of acetylanilide, as well as of the amides. 



ON THE SUPPOSED PRE-CAMBRIAN ROCKS 

 OF ST. DAVID'S 1 



HTHE author began by briefly narrating the circumstnnces 



under which he had been led to study the geology of St. 



David's. He had visited the district twice — first in company 



1 Abstract oi a paper read at the Geological Society by Archibald Geikie, 

 F.R.S. 



with Mr. B. N. Peach, with whose cooperation nearly all the 

 field work was done, and again in conjunction with Mr. W. 

 Topley. The paper was divided into two parts, the first being 

 mainly controversial, and the second descriptive. 



According to Dr. Hicks, there are at St. David's three dis- 

 tinct pre-Cambrian formations : the "Dimetian," consisting of 

 crystalline, gneissic, and granitoid rocks; the " Arvonian," 

 formed of felsites, quartz-porphyries, halleflintas, and other 

 highly-silicated rocks ; and the " Pebidian," composed of tuffs, 

 volcanic breccias, and basic lavas. He regards the " Arvonian " 

 a- later than and unconformable to the " Dimetian," and the 

 " Pebidian " as younger than, and unconformable to both ; and he 

 nsserts that the basement conglomerate of the Cambrian system 

 lies quite unconformably on all these rocks, and is in great part 

 made up out of their waste. 



Taking up each of these groups in the order of sequence 

 assigned to them, the author maintained that the "Dimetian 

 group" is an eruptive granite, which has disrupted and altered 

 the Cambrian strata, even above the horizon of the supposed 

 basal conglomerate. He described a series of natural sections 

 where this relation is exposed, particularly one on the coast at 

 Ogof-I le-ugn, where the cmglomerate has been torn off and 

 involved in the granite, and has been intensely indurated, so as 

 to bee ime a kind of pebbly quartzite. No other rock occurs 

 within the granite mas except dykes of diabase, which rise 

 through all the rocks of the district, but are especi illy abundant 

 in the granite. The veins of finer granite, so general in granite 

 areas are conspicuous here. In short, whether studied in hand 

 specimens or on the ground, the rock is so unmistakably an 

 eruptive mass that the author could not understand how this 

 view, which was that expressed on the Geological Survey maps, 

 should ever have been called in question. The manner in which 

 it has risen across the bedding of successive horizons in the 

 Cambrian series proves tha', in-tead of being a pre-Cambrian 

 gneiss, it must be much younger than all the Cambrian rocks of 

 the district. 



The " Arvonian group'' consi-ts of quartziferous porphyries, 

 or elvans, associated with the granite, and of the metamorphosed 

 strata in their vicinity. Reference was made to natural sections 

 where the actual intrusion of the elvans across the bedding of 

 the rocks could be seen. 



The "Pebidian group" comprises a series of volcanic tuffs 

 and breccias, with ii.terstratified and intrusive lavas. The author 

 maintained that this group forms an integral part of the 

 Cambrian system as developed at St. David's. It has been 

 broken through by the granite and porphyries, and is therefore 

 of older date. Instead of being covered unconformably by the 

 Cambrian conglomerate, as asserted by Dr. Hicks, the volcanic 

 group is over ain quite conformably by that rock ; and seams of 

 tuff are interstratified with the conglomerate and occur on various 

 horizons above it. The conglomerate, instead of being mainly 

 composed of fragments of the rocks beneath it, consists almost 

 entirely of quartz and quartzite, only 4 per cent, of fragments 

 having been found to have been derived from some of the pro- 

 jecting lava islands underneath it. 



From the evidence now brought forward, the author contended 

 that as the names "Dimetian," " Arvonian," and " Pebidian" 

 had been founded on error of observation, they ought to be 

 dropped out of geological literature. 



In the second part of his paper the author gave the results of 

 the survey which he had made of the district with Messrs. Peach 

 and Topley, and of his study of a series of more than 100 thin 

 slices of the rocks collected at St. David's. He found that he 

 could corroborate generally the descriptions of previous writers 

 on the microscopic structure of the rocks, and that investigation 

 with the microscope amply confirmed the deductions he had 

 drawn from observations in the field. 



I. Order of Succession of the Rocks. — The following rock- 

 groups in the Lower Cambrian series are recognisable at St. 

 David's, and are given in descending order : — 



4. Purple and greenish grits, sandstones, and shales. 

 3. Green and red shales and sandstones, with thin tuffs 



(Lingulella prirrueva). 

 2. Quartz comglomerate. 

 1. Volcanic group (tuffs, schists, lavas). 



The volcanic group forms the oldest part of the Cambrian 

 series at this locality. The bottom is not reached, but about 

 1800 feet are visible. It consists mainly of purplish-red, green, 

 grey, and pale tuffs, with occasional breccias and bands of 

 olivme-diabase. Analyses of some of these rocks had been 



