388 



NATURE 



[Feb. 27, 1879 



Kopp first proceeded to sketch the methods employed for 

 determining the molecular and atomic weights of the 

 elements. When an element can be volatilised con- 

 veniently, so that we can obtain its vapour-density, its mole- 

 cular weight is readily decided. Those elements which 

 enter with a large number of volatile, or gaseous bodies, 

 like carbon, present but little difficulty. Those, like zinc, 

 which form but one class of volatile compounds, leave 

 much to be desired, for a series of homologous bodies are 

 no better than a single member of the series. In this 

 case, however, the specific heat of the element comes to 

 our aid, and we can usually take such a multiple of its 

 equivalent as will give, when multiplied by the specific 

 heat, a product not far from six. Naumann' s law also 

 enables us to make use of the specific heat of salts as 

 well as that of the elements, the product of the specific 

 heat by the sum of the atomic weights being nearly equal 

 for similar compounds, and usually six times that of the 

 number of atoms in a molecule. But this fails in some 

 cases, probably, because we cannot take the specific heat 

 at a sufficiently high temperature, as in the case of ice. 

 In many cases where the above tests fail, isomorphism 

 holds good. But totally unlike bodies, containing an 

 unlike number of atoms in the molecule, have the same 

 crystalline form. To avoid this dilemma, Prof Kopp 

 proposes to limit the term isomorphous to those com- 

 pounds which possess the same crystal-forming power, as 

 proved by their ability to crystallise together, or, if 

 unequal in solubility, the ability of one crystal to grow in 

 a solution of the other. Both of these cases were beauti- 

 fully illustrated by alums. If a trace of a chromalum 

 solution be added to a solution of potash-alum, each 

 crystal that forms will contain both, as shown by the 

 reddish tinge, and the colour deepens as the quantity of 

 chromalum added is increased. On the other hand, 

 when a crystal of one sort of alum is placed in a solution 

 of the other kind, it continues to grow. Fine specimens 

 of such crystals were exhibited by the speaker, who 

 is remarkably skilful in this matter of growing and 

 nursing crystals. Many other isomorphous salts were 

 exhibited, such as the sulphates of magnesia and nickel ; 

 in some cases two different salts had been deposited 

 alternately over the crystal of a third salt. Most inter- 

 esting were rhombohedra of calxspar covered with nitrate 

 of sodium, thus proving these two bodies isomorphous. 

 The professor acknowledged that he had had much dif- 

 ficulty in obtaining these, and had utterly failed to make 

 a crystal of arragonite grow in a solution of nitrate of 

 potash. Prof. Kopp said, in conclusion, that unlike 

 number of atoms could not replace each other in a mole- 

 cule of two isomorphous bodies. Sulphate of cadmium 

 will crystallise with eight equivalents of water to three of 

 the anhydrous salt. Sulphate of didymium crystallises 

 with eight equivalents of water. Both have the same 

 crystalline form, but two atoms of didymium seem to re- 

 place the three of cadmium : — 



DigSsOia -f 8H,0 ; 



CdsSsUia -f- 8H2O. 



But these salts will not crystallise together, and crystals 

 of the latter, from a mixed solution, contain no pinkish 

 tinge of didymium. 



HER MAJESTY'S ASTRONOMER AT THE 

 CAPE 



M- 



R. DAVID GILL has been gazetted successor to 

 Mr. E. J. Stone in the direction of the Royal Ob- 

 servatory, Cape of Good Hope. The discrimination 

 exercised by the First Lord of the Admiralty in this 

 appointment, we are confident will be appreciated and 

 applauded by astronomers generally. Obtaining his first 

 experience in practical astronomy in the Observatory at 

 Aberdeen, and in a private observatory which he erected 



in the same place, Mr. Gill was so fortunate as to be 

 associated with Lord Lindsay in the designs and details 

 of the large observatory founded by this nobleman at Dun 

 Echt in 1870, taking the position of chief of the staff. He 

 thus became engaged in the organisation of the expedition 

 to the Mauritius fitted out by Lord Lindsay for the obser- 

 vation of the transit of Venus, on which occasion advantage 

 was taken of the circumstance of a heliometer forming part 

 of the equipment to determine the sun's distance by mea- 

 sures of the planet Juno, being the first trial of the method, 

 and attended with satisfactory results ; the details of this 

 work were published by Lord Lindsay as the joint work 

 of himself and Mr. Gill. In connection with the same 

 expedition, Mr. Gill arranged and personally conducted 

 the whole of the chronometric and telegraphic longitude 

 determinations connecting Berlin, Malta, Alexandria, 

 Suez, Aden, Bombay, Seychelles, Reunion, Mauritius, 

 and Rodriguez. It was while engaged upon these opera- 

 tions that he undertook, at the request of the Khedive, 

 the measurement of the first base line of the geodetic sur- 

 vey of Egypt. In 1877 Mr. Gill laid before the Royal Astro- 

 nomical Society a proposal to determine the sun's distance 

 by heliometric observations of the planet Mars about the 

 very favourable opposition of that year. Lord Lindsay 

 lending his heliometer for the purpose. The proposal 

 met with the support of the Astronomer-Royal and 

 Council of this Society, and was further aided in its 

 execution by a grant from the government funds in the 

 hands of the Royal Society. The Island of Ascension 

 was fixed upon as a favourable station for these observa- 

 tions, and Mr. Gill proceeded to Ascension in June, being 

 occupied there about six months in the necessary prepa- 

 rations and carrying out of the scheme. The reductions 

 are still proceeding, but in proof of the importance 

 attached to this attempt to obtain a reliable value of the 

 solar parallax and the interest felt by the leading astro- 

 nomers of different nations in his work, it may be men- 

 tioned that on asking for aid in the accurate determina- 

 tion of the positions of the stars observed with Mars, 

 his request was cordially acceded to at the following obser- 

 vatories : — Greenwich, Oxford, and Liverpool, Albany, 

 U.S., Berlin, Cambridge, Mass., Cordoba (the national 

 establishment of the Argentine Republic), Konigsberg, 

 Leipsic, Leyden, Melbourne, Paris, Pulkova (the Imperial 

 Observatory of Russia), and Washington. 



We will express the hope that Mr. Gill may carry to 

 his new sphere a continuance of the great energy he has 

 hitherto shown and repeat our conviction that his nomina- 

 tion by the First Lord to the important position of " Her 

 Majesty's Astronomer at the Cape," will be hailed with 

 great satisfaction in the astronomical world. It is under- 

 stood that Mr. Gill leaves England early in May, arriving 

 at the Cape in good time to confer with Mr. Stone upon 

 the future work of the Observatory. 



OUR ASTRONOMICAL COLUMN 



The Naval Observatory, Washington. — The Re- 

 port of Admiral Rodgers, superintendent of this great 

 astronomical' establishment, for the year 1878 has just 

 been issued. The operations of the institution have been 

 more than usually extended, involving expeditions for 

 the observation of the transit of Mercury on May 8, and 

 the total solar eclipse of July 29. The 26 inch refractor 

 has been in charge of Prof. Asaph Hall, with Prof. 

 Holden as assistant, and has been constantly employed 

 in the observation of sateUites, double stars, and nebula;, 

 and occasionally of comets. Admiral Rodgers mentions 

 that many foreign astronomers visiting the United States 

 on the occasion of the eclipse, took the opportunity of 

 inspecting this instrument, expressing very generally an 

 opinion that the mounting was too light, and in this 

 opinion the superintendent to a certain degree coincides, 



