464 



NA TURE 



[March 14, 1901 



so little mention is made of metallurgical methods of 

 assaying, which are frequently employed, not only for 

 estimating silver and gold, but in the technical analysis 

 of lead and copper ores. J. B. C. 



Recueil de Donnees numeriques. Optique. By H. Dufet 

 Part iii. Pp. 787-1313. (Paris : Gauthier-Villars, 

 1900.) 



The second part of this invaluable collection of 

 physical and chemical data dealt with the optical proper- 

 ties of solids, and has already been described in these 

 columns (vol. Ix. p. 28). The present (and concluding) 

 volume contains tables showing the rotatory powers of 

 crystalline bodies, liquids and solutions, interference 

 colours produced when rays of white light strike nor- 

 mally upon a layer of air of a given thickness, and sup- 

 plementary tables of refractive indices, standard wave- 

 lengths, optical properties of inorganic and organic 

 bodies, and other data. In all cases full reference is 

 given to the authority for the values tabulated. The 

 Physical Society of France has done a service to science 

 by arranging for the preparation and publication of 

 these results, which have been gathered from many 

 sources, and are frequently difficult of access. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications. \ 



Apatite in Ceylon. 



A PARAGRAPH in Pfof. Miers' interesting notice (Nature, 

 March 7) of the results of Dr. Griinling's mineralogical expedi- 

 tion to Ceylon may convey the impression that Dr. (kunling was 

 the first person to recognise, and Dr. Schiflfer the first to analyse, 

 the sky-blue apatite of Ceylon. 



Now Mr. H. Willett, of Brighton, handed me several* years 

 ago a beautiful transparent specimen which I at once identified 

 as apatite though its colour was unusual. The sky-blue, prism - 

 atically developed crystals were embedded in a white dolomitic 

 matrix. This specimen had been brought from Ceylon by Mrs. 

 Willett. Its exact provenance was unknown, but some spot 

 not far from Kandy and the railway thither was suggested as the 

 probable locality. There can be little doubt that this specimen 

 came from the neighbourhood of Wattegama, between Kandy 

 and Matale, where Dr. Griinling obtained the apatite analysed 

 by Dr. Schififer. In the Mineralogical Magazi7ie iox April 1899 

 (p. ix) will be found a notice of a paper read by me before the 

 Mineralogical Society on January 31, 1899, just two years ago, 

 entitled "Analyses of Ceylon Apatite." Following this title 

 occurs the paragraph : "Prof. Church exhibited blue apatites 

 from Ceylon, one containing as much as 3 "21 per cent, of 

 chlorine, others only 0-63 and 0*34 per cent." The last of these 

 three figures represents the percentage of chlorine found by me 

 in Mr. Willett's specimen. 



In discussing, in the autumn of 1898, the question of Ceylon 

 apatite with Prof. Judd and Mr. F. W. Rudler, I found that the 

 identification of this mineral did not seem at that time to have 

 been published. Prof. Judd was good enough to supply me 

 with specimens from another Ceylon locality (near Newara 

 Eliya) and having a different matrix. Here the colour of the 

 apatite was paler and its crystalline habit indistinct. This 

 sample gave 0"63 per cent, of chlorine. 



But by far the most interestmg specimens were some imperfect 

 tabular crystals sent to me in 1898 by a gem-merchant of 

 Colombo. These were of a much richer blue colour than any of 

 the other examples. Indeed, my Colombo friend's attention had 

 been drawn to them by one of his " Moormen " having brought 

 to him, some time previously, a perfectly transparent cut gem 

 weighing five carats, which he offered as a blue spinel ! I wrote 

 for uncut specimens, which were soon procured from the same 

 Moorman, but he had removed them from the matrix. The 

 locality of these specimens proved to be Avisavelle. One of 

 these crystals it was that gave me 3*21 as the percentage _of 



N'O. 1637, VOL. 63] 



chlorine. I may add that the dichioism of this variety closely 

 resembled that of vivianite, 



I would remark in conclusion that the number of Dr. Groth's 

 Zeitschrift fiir Krystallographie u. Mineralogie in which the 

 blue apatite of Ceylon is described was published last year, long 

 after the notice of my paper had appeared in the Mineralogical 

 Magazine. If any earlier communication on the subject has 

 appeared I should welcome a reference to it. 



A. H. Church. 



Maps in Theory and Practice. 



Every advanced treatise on astronomy defines and explains 

 certain kinds of map projection ; but in all these accounts I have 

 been struck with the absence of any notice of the particular kind 

 of projection with which we are most familiar — the kind usually 

 employed for representing the world in hemispheres ; in fact, the 

 commonest kind of map projection. 



I have for the first time come across a notice of it, at p. 126 

 of the February number of the Americajt Journal of Science, in 

 the following terms : — • 



" The method of projection almost universally employed by 

 geographers for representing hemispherical surfaces is the so- 

 called globular projection, invented in 1660 by the Italian 

 Nicolosi (Germain, ' Traite de Projections des Cartes Geo- 

 graphiques,' Paris, about 1865). The equator is divided into 

 equal parts, and the meridians are circular arcs uniting these 

 points with the poles. The parallels are likewise circular arcs, 

 dividing the extreme and central meridians into equal parts." 



As three points suffice for determining a circle, this definition 

 is complete for the mapping of a hemisphere extending from 

 pole to pole. To apply it to a smaller portion of the earth's 

 surface, let the hemisphere be so taken that this portion is 

 centrally placed between east and west ; then the central 

 meridian will be straight. 



As it is desirable that theory should be kept in touch with 

 practice, I would commend this subject to the attention of 

 teachers of geography and astronomy. J. D. Everett. 



II Leopold Road, Ealing, W., March 11. 



Early Observations of Volcanic Phenomena in 

 Auvergne and Ireland. 



Mr. G. p. Scrope (" Extinct Volcanos of Central France," 

 ed. 2, 1858, p. 30) describes how Guettard and Malesherbes, 

 returning from Italy, met Faujas de St. Fond at Montelimar in 

 1751, and, in his company, founded the theory of the volcanic 

 origin of the mountains in the Vivarais. Guettard and Male- 

 sherbes thence proceeded to Auvergne, where M. Ozy, a chemist 

 of Clermont-Ferrand, acted as their guide. Ozy knew his 

 ground well, and had already observed the general aspect of the 

 rocks, since Guettard (Mem. Acad. roy. des Sciences, 1752, 

 p. 37) says that he " m'assura . , . que je trouverois par-tout 

 une meme structure & les memes matieres qu'il m'avoua ingenu- 

 ment n'avoir jamais reconnues pour ce qu'elles etoient." 



Sir A. Geikie has justly written of the discoveries of Guettard 

 (" Anc. volcanoes of Gt. Britain," preface), " To France, which 

 has led the way in so many departments of human inquiry, 

 belongs the merit of having laid the foundations of the systematic 

 study of ancient volcanoes." 



Considerable interest, therefore, attaches to a letter from Ozy, 

 published by Faujas de St. Fond in 1778 (" Recherches sur les 

 volcans eteints du Vivarais," p. 434), and written in reply to 

 inquiries as to what authors had first visited the volcanoes of 

 Auvergne. Ozy refers somewhat casually to his meeting with 

 Guettard, which was already well known through the inform- 

 ation provided by him, and fully acknowledged in Guettard's 

 paper. But he states that a year before, that is, in 1 750, he 

 was visited by ".Olzendorff," an Englishman, and " M, Bowls, 

 irlandois," who came to inspect the adjacent lead-mines. He 

 continues : " Nous montames ensemble au Puy de Dome, & ce 

 fut la que j'appris pour la premiere fois a connoitre les crateres^ 

 les laves, &c. , car auparavant je n'etois pas plus instruit sur cet 

 objet que les autres habitans de cette province." 



It seems hardly possible to construe this passage, written in 

 answer to a direct inquiry from St. Fond, into a Confusion of 

 the two ascents. The "ensemble" refers to Olzendorff and 

 Bowls ; " la premiere fois " can hardly refer to events of the 

 subsequent year. "Bowls," moreover, was, with high prob- 

 ability, William Bowles the mineralogist, who is known to have 



