Jan. i, 1880] 



NATURE 



and redetermination of some forms involving very high 

 indices on crystals examined by the late M. Hessenberg. 

 The measurements -were made with one of the goniometers 

 devised by Prof. Groth, which gives definite results when 

 the faces are good. The collimator and telescope are 

 fixed, however, at right angles to one another, so that the 

 determination of striated faces, such as many of those 

 examined were, is difficult and uncertain. Much more 

 reliable measurements would be obtained were the angle 

 between collimator and telescope small, and it would be 

 very easy to arrange the collimator so that the angle of 

 incidence and reflection might be varied at will. Prof. 

 Miller used to arrange his goniometer so that the angle 

 between the incident and reflected ray was less than 20 3 , 

 and was thus able to get rid of a good deal of the diffi- 

 culty arising from striation. 



Mr. Irby has guarded himself from error by the com- 

 parison of several independent observations of the angles 

 made by a new face with those adjoining it, with the 

 angles obtained by calculation, and has avoided employ- 

 ing the angles made with faces on more distant parts of 

 the crystal, though the latter would be often better adapted 

 for purposes of calculation. He criticises Prof, vom 

 Rath's method of observation by taking the reflection of 

 a window-bar as signal. The error which would thus 

 arise would not exceed 1' in the case of good faces, and I 

 believe Prof, vom Rath only employs this method of 

 observation with very good faces. Another source of 

 error would be due to the proximity of the signal which 

 would give a considerable error if the edge were not well- 

 centred. Moreover, a goniometer with vertical plane of 

 reflection is very difficult to get into or keep in good 

 adjustment, and errors might arise in this way. None of 

 these errors will, however, account for the impossibility of 

 getting simpler indices for the form {35, 17, 32}, consider- 

 ing how definite were the angles obtained from the several 

 faces of the scalenohedron. A careful criticism of this 

 form at the time it was published, and of all the different 

 ways in which errors might be piled up in the course of 

 the analysis, failed to lead to any result but that of 

 admitting the possibility of forms with these high indices. 

 In the Cambridge collection is a crystal of quartz with an 

 extremely well-developed face, which Prof. Miller has 

 determined to be {50, 19, 19}. It is very slightly rounded 

 on the edge of the prism face. Of course, when the faces 

 are rounded or otherwise distorted, indices calculated 

 from the observations are mere approximations. Seeing 

 the great variations which occur in the angles of well- 

 crystallised minerals, good work might be done in testing 

 the constancy of the angle of the cleavage rhomb in the 

 specimens from different localities. Breithaupt's deter- 

 minations of this angle are unfortunately not sufficiently 

 reliable. 



LETTERS TO THE EDITOR 



[The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, or 

 to correspond with the writers of, rejected manuscripts. No 

 notice is taken of anonymous communications. 



[The Editor urgently requests correspondents to keep their letters as 

 short as possible. The pressure on his space is so great that it 

 is impossible otherwise to ensure the appearance even of com- 

 munications containing interesting and novel facts .] 



The Molecular Velocity of Gases 

 Your correspondent, M. Hajnis, asserts in a foot-note ap- 

 pended to his letter published in Nature, vol. xxi. p. 176, that 

 "the formula for the molecular velocity (of gases) was first given 

 by Kronig," and not by Joule. I am at a lo--s to understand 

 how this statement can be justified. 



Kronig's paper appeared in 1S56, while Joule's calculation, 

 which is that now generally received, is of date 184S. In his 

 discourse on molecules (Phil. Mag., December, 1S73), Prof. Max- 

 well says : " The further development of the theory is generally 



supposed to have begun with a paper by Kronig, which does not, 

 however, so far as I can see, contain any improvement on what 

 had gone before. " R. 



Weaver Birds and Fire-Flies 



Under the heading "Natural History Notes from Burmali," 

 in Nature, vol. xx. p. 362 of the present series, Mr. R. 

 Romanis asked if any of your readers have ever seen or heard of 

 weaver birds sticking fire-flies to lumps of mud on the sides of 

 their nest for the purpose of illumination. 



The tradition that certain birds of the weaver family (PUttiiU) 

 and their allies do this, is prevalent over a large portion of the 

 globe inhabited by these birds. I have traced it personally from 

 China, all parts of India, Burmah, Ceylon, the Malayan 

 Peninsula, Indian Archipelago, Southern, Eastern, and Western 

 Africa, and South America. I have examined "weaver birds' " 

 nests from all these countries, and have found lumps of mud 

 sticking inside them, and "therefore it must be true, you knoi • .' ! " 



But for what use are these lumps of mud stuck there? 



Some years ago I wrote an article in the Cape Monthly 

 on this very subject, entitled "Strange Stories and 

 their probable Origin," and I started by saying "Where there 

 is smoke there must be fire," quoting the old proverb. 



My belief is that these lumps of mud are used as scrapers 

 on which to clean the birds' bills, as I have frequently found the 

 wing ca-es, and other dibris of Colcoptera, &c. , fixed to them. 

 Hence the superstition that they stick fireflies thereon. I should 

 remind your readers that all the "weavers " are grain feeders, ami 

 perhaps only occasionally partaking of insect-food, they arc 

 bothered by the bits sticking to their bills. I see my tamed 

 birds are most careful in cleaning their beaks. 



At the time I wrote my article above alluded to, I was not so 

 conversant with the African "hang-nests" as I afterwards became, 

 but I can affirm that in all the places I have named the 

 tion, and the mtui, is to be found. E. L. 1 



British Consulate, Noumea, October 22, 1S79 



The Papau 



Permit me to add to my friend the Rev. S. J. Whitmee's 

 testimony of the papau being eaten by birds in the Samoan 

 Islands, that it is here (New Caledonia) a favcurite food of the 

 " white eye " (Zoslerops), and in the Loyalty Islands was used as 

 the only bait to attract these birds, of which dozens were 

 brought me — of the three known species of that genus which 

 inhabit that group (see my letters to Field newspaper) all caught 

 by the boys through its means. If my memory serves me 

 rightly, I have seen the papau in Mauritius eaten by a species of 

 Zosterofs. I . Layard 



British Consulate, Noumea, New Caledonia, 

 October 22, 1879 



Scale of Colours 



In Nature, vol. xxi. p. 172, it is stated that at a meeting of 

 the Zoological Society, December 2, 1879, " A letter was read 

 from Mr. E. L. Layard, F.L.S., advocating the desirability of 

 a fixed scale of colour for use among naturalists in describing 

 the plumage and pelages of birds and other animals." Perhaps 

 Mr. Layard is not aware that such a scale, in form of thin Svo, 

 was published by Fatrick Syrne in Edinburgh, in 1S21, the tints 

 being illustrated by carefully coloured examples. The exact 

 title of the work, a copy of which is in my c\\ 11 library, is as 

 follows : — " Werner's Nomenclature of Colours, with Additions, 

 arranged so as to render it highly useful to the Arts and Sciences, 

 particularly Zoology, Botany, Mineralogy, and Morbid Anatomy. 

 Annexed to which are Examples selected from well -known Ob- 

 jects in the Animal, Vegetable, and Mineral Kingdoms." The 

 date given above is that of the second edition, which was " Printed 

 for William Elackwood, Edinburgh, and T. Cadells, Strand, 

 London." L. Elomefield 



Bath, December 22, 1879 (late Jenyns) 



On the " Habitat " of Lophiomys 



On reading the review of " Cassell's Natural History," vol. iii., 



given in Nature, vol. xxi. p. 136, I find that both the author 



and the reviewer do not appear to have been aware that the 



"habitat" of that most interesting rodent, Lophiomys im/'tausi, is 



