OCTOMKR 24, 1895] 



NA TURE 



619 



be found on comparing pp. 191 and 313. On p. 191 we 

 have a short abstract of an article on petroleum, by A. Riche 

 and O. Ilalphen. On p. 313 we have a long abstract of the 

 same article. In one case it is given under i/iialitalive organic 

 chemistry, in the other under ijuaiilila/ivc organic chemistry. 

 Net the reference in each case is the same — "y. I'liarm. Chim., 

 1894, 30, 289." In this case, therefore, the abstracts are not 

 even prepared from dift'erent journals. 



I would suggest, then, that the first reform which the Editing 

 Committee might institute in carrying out their scheme of re- 

 trenchment, should be one placing a limit on the number of 

 abstractors who are to deal with one and the same article, even 

 when it occurs in different publications. Jamks Henurick. 



<ilasgow, ( )ctol>er 2. 



Note on the Dendrocolaptine Species, ' Dendrexe- 

 tastes capitoides" of Eyton. 



It recently became necessary for me co examine some of the 

 Dendrocolaptine birds in this museum, and among them the 

 species nametl above. Our specimen, the type of the genus 

 nendrexetastcs iaanAcC^ by Eyton in 185 1, in Jardine's "Con- 

 tributions to Ornithology," on a skin from an unknown 

 locality, formerly in Lord Derby's museum, has evidently been 

 examined by Dr. Sclater, for its label bears, in the well-known 

 calligraphics of that distinguished authority on this group, the 

 name Deitdrtwetaslcs Ifiitniiiicki. The difficulty I have in 

 ascribing our sjiecimen to thai species is the cause of this note. 

 .\ccording to the fifteenth volume of the " British Museum 

 Catalogue of Birds," by Dr. I'. L. Sclater, the genus con- 

 tains but two species. D. temmiiicti and D. dcvillii, which, by 

 his key on p. 140, are distinguished from each other, the former 

 by having *' blackish cross-bands" on the belly, and the latter 

 having that region " uniform brown." On consulting Eyton's 

 original tiescription in the " Contributions to Ornithology," I 

 can find no mention of any cross-bands on the belly ; for there 

 are none on the skin, which is apparently that of a mature 

 bird. In looking up next the description by Lafres- 

 naye, in the " Revue de Zoologie " for 1851. of his D. tern- 

 iiiinckii, to which Dr. Sclater has relegated as a synonym 

 Eyton's D. capitoides, I read : — " .... pectoris vent risque 

 plumis totis umbrinis, in medio macula triangulari-elongata 

 nivea nigro marginata notatis : ventris niaculis strictis ; fere 

 linearibus ; subcaudalibus pallide rufescentihus, albo late, 

 fuscoque angusle vittatis. . . ." These words, as I interpret 

 them, make no mention of the presence of cross-bands on the 

 belly of /A taiimincki, while the latter half of the quotation, in 

 regard to the under-tail-coverts being pale rufous, with broad 

 white and narrow fuscous spots, does not apjily to D. capitoides, 

 for the type-skin before me presents no such characters. The 

 ])late illustrating Lafresnaye's description of the first-mentioned 

 bird (loc. sup. cit.) shows its breasl-spots to be much narrower, 

 though not linear, and shorter than those in D. capitoides, while 

 the spots on the feathers on the upper part of the belly can 

 hardly be termed " ferelinearibus," which they are, however, in 

 D. capitoides. The lower belly in the plate, " plumis totis 

 lunbrinis," shows, just as in the last-mentioned species, not a 

 single cross-band. It would appear to me, therefore, that D. 

 tapitoides, Eyton, can scarcely be = D. ttiiiiiiincki, Lafr. , while 

 the latter differs from D. devillii (of which I regret our museun. 

 does not possess a specimen), and, I take it, from D. capitoides, 

 by its smaller and narrower throat-spots. The subcaudal 

 characters separate D. capitoides from D. teinmincki, and 

 apparently the typical D. de-oillii is separated from it also by 

 the "striisstrictissimis" of the breast, and the very linear shaft- 

 stripes of the uj^per neck feathers. Is D. capitoides — D. 

 devillii? Or are there three species? I incline to the 

 "pinion that (here are. IIf.nrv O. Forbes. 



The Museums, Liverpool, October 8. 



The Pressure of a Saturated Vapour as an Explicit 

 Function of the Temperature. 



Ir may be i^f some interest to mite that Cailletet and Mathias' 

 "Law of Diameters," in combination with any equation of state, 

 such as Van der Waals', which applies to the region of coexist- 

 ence of liquid and vapour, supplies an (empirical) expression for 

 the maximun) pressure of a vapour at any tenqx-rature T in the 

 form of an explicit function of this tem])erature and known 

 constants. 



NO. 1356, VOL. 52] 



Let p, V and T denote the pressure volume and absolute 

 temperature of unit mass of the substance. According to \an 

 der Waals' original equation of state, we have then : — 



(/ + $) (-" - *) = RT. 



If Z'j, <•», z'j denote the roots of this cubic in v, we ha\ e : — 



RT 



z'l -(- I.J -f r, = * -i- !ii. (i.) 



^'I's + ^s^'s + r'3''i = -- (ii.) 



/ 



ab .... , 



7.-1^,2/3 = (in.) 



P 

 Now, for any definite value of T less than the critical tempera- 

 ture, these equations give, when we put / equal to the maximum 

 vapour-pressure corresponding to this temperature, three values, 

 f'l, J'o, 7'3, two of which (say -i\ and <'._,) denote the volumes of 

 unit mass of the substance in the states of saturated vapour and 



"saturated" liquid at this temiwrature. Accordingly, - and 



~ denote the densities of the substance in these states, and the 



law of Cailletet and Mathias, above referred to, enunciates that 

 the arithmetic mean of these densities can be very fairly repre- 

 sented as a linear function of the temperature. Therefore we 

 can write : — 



'"—^ = ^X (iv.) 



where if> denotes a .linear function, whose two constants arc 

 know-n. 



Eliminating r',, ->.,, and v.^ from the four equations (i.), (ii), 

 (iii.)and (iv. ), we readily obtain : — 



p = RT(i - ^0T) - «»T(i -ipT)-^ _ ^^.^ 



This result simply amounts to the following : — 



If we fix the temperature T of a vapour, then the maximum 



vapour-pressure at this temperature is completely determined, 



i.e. 



P = l'(T). 



Similarly the sum of the densities of saturated vapour and 

 liquid in contact with it is determinate if T is fixed, and thus 



-L H- i_= ^(T). 



Equation (v.) shows that the former function is known if the 

 latter be known, and as Cailletet and Mathias have shown that 

 the latter is very approximately linear, we can give the form of 

 F(T). 



This result, however, is not of any practical use unless the 

 equation of state does really apply with good approximation to 

 the region of liquid and vapour. I'". G. Donnan. 



Ilolyw'ood, Co. Down. 



Colours of Mother-of-Pearl. 



In numerous text-books the colours of mother-of-pearl are 

 included amongst phenomena of colour produced by striated 

 surfaces, and though it is conceded that only a part of the colour 

 is due to this cause, that part is generally assumed to lje, at any 

 rale, an appreciable quantity. Experiment will show, however, 

 that such is not the case. When the colour produced by the 

 striations is viewed in an impression "->f the jx'arl on sealing-wax 

 or gelatine it is visible, though it is totally <lifferenl in ch.aracter 

 from the iridescence of the pearl itself, in which the liny con- 

 tribution of colour from the striations is completely overpowered 

 by that due to another cause. In white mother-of-pearl the 

 striations are often its close together as in coloureil varieties, and 

 at certain angles, when viewed by light from a definad source, 

 there is a little colour visible in the white siwcimens ; jnst somuch, 

 and no more, is contributed by the striations of the coloured 

 specimens, as may be shown by viewing a piece under the 

 surface of water, when the effect of the striations is necessarily 

 abolished, though the iridescence is not at all appreciably 

 diminished. The whiteness of some varieties must be attributed 

 to a different thickness or greater opacity of the lamina;. It is 



