Sept. 5. 1S7S] 



NATURE 



487 



of Plymouth," is valuable and readable, containing the 

 results of careful observation. " A Catalogue of the 

 Geometrina of Plymouth and its Vicinity," by Mr. G. C. 

 Bignell, will interest entomologists. " Our Obligation to 

 Greek Thought," by Prof. Chapman, is a thoughtful 

 paper, and there are several other excellent papers of 

 historical and antiquarian interest. 



Vidcanologische Studien. Von Dr. Eduard Reyer. (Wien • 

 1878.) 



We have already had occasion to direct attention to the 

 valuable contributions to the theory of volcanoes which 

 have recently been published by Dr. Reyer, of Vienna. 

 The memoir before us fully maintains the reputation of its 

 author as an able investigator and original thinker. In 

 it he discusses the nature of the materials which remain 

 in the throat and deeper portions of a volcanic vent, after 

 the eruptive action has ceased, and the features presented 

 by those volcanic cones which are formed not by violent 

 explosive action but by the quiet outwellingof liquid lava. 

 Dr. Reyer's remarks on both these questions will be found 

 to be eminently valuable and suggestive. J. W. J. 



LETTERS TO THE EDITOR 



YThe EdiUr does not hold himself responsible for of inions 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. "l 



Is the Sun One-sided .' 



When Broun and Homstein detected the existence of a 

 terrestrial magnetic inequality, of which the period is nearly 

 that of the sun's rotation, it was natural to regard this inequality 

 as a direct result of the rotation of our luminary. Nevertheless, 

 there are grave reasons against this hypothesis. In the first 

 place, it is extremely difficult to imagine the sun to be one-sided 

 in its magnetic influence. From what we know of our luminary, 

 it must in a great measure be composed of gaseous matter, of 

 which the outer layers are in violent motion, so that we can 

 hardly imagine one meridian to be permanently different from 

 another. 



Another objection is derived from the fact that the period of 

 this peculiar magnetic inequality (of whose existence there can 

 be no doubt) is very nearly that of the sun's rotation in space, 

 and decidedly less than that of its synodic rotation. Now, if 

 we can imagine the sun to affect the earth in this peculiar 

 manner, we should be inclined to suppose that the period of 

 such influence would be that of its synodic rotation, that is to 

 say, of its rotation with regard to the earth. 



But if this inequality be not due to the sun's rotation, how (it 

 may be asked) is it possible to account for it ? 



In the first place there is accumulating evidence of the exist- 

 ence of an intra-mercurial planet with a time of revolution, not 

 differing greatly from that of the sun's rotation. Again, there 

 can be little doubt that we have various magnetic inequalities, 

 of which the periods are the same as those of the most pro- 

 minent planetary configurations. May not, therefore, this mag- 

 netic inequaUty be connected with the intra-mercurial planet of 

 the existence of which we are becoming sure, and is it not 

 possible that a discussion of magnetic changes may aid us in 

 settling this very interesting and important question ? 



St. Andrews, August 26 Balfour Stewart 



Von Cotta's " Geologic der Gegenwart " 

 Your journal contains, in the number for August 8, a short 

 notice of my newly- published "Geologic der Gegenwart," with 

 a signal misrepresentation of the coloured frontispiece, begin- 

 ning with the lines, "We can scarcely regard," to the end of 

 the paragraph. 



The illustration in question really is a representation of the 

 fact that igneous rocks — both volcanic and plutonic — being 

 originally products of the ceqtral parts of earth, form differen't 



species, according to their being rich in silica (acidic rocks) or 

 poor m sdica (basic rocks), and according to their consolidation 

 at a considerable depth as plutonic, or nearer the surface as 

 volcajiic rocks. Becoming solid at a great depth, the acidic 

 fluids have formed granites or porphyrites ; nearer the surface, 

 trachytes or trachytic lavas. The basic fluids, on the other hand, 

 became syenites or greenstones at a great depth, and basalts or 

 basaltic lavas nearer the surface. 



All these various rocks by no means belong to particular 

 geological periods, but at all times have either overflowed or 

 penetrated other formations, and been accompanied by tuff 

 formations. 



The misapprehension contained in yom- notice is doubly pain- 

 ful to me, because I think I have been the first German geologist 

 who decidedly adopted Lyell's view with regard to the utter 

 independence of the nature of rocks from their geologic age : 

 witness the first edition of my "Geologic der Gegenwart,'' 

 pubhshed in 1866. Bernhard v. Cotta 



Freiberg 



On the Wax of Pceciloptera 



A species of Pceciloptera was this year rather common in the 

 neighbourhood of this city, covering thickly the branches of 

 Cassia obtusifolia, L., and more sparingly of Cassia spectabilis, 

 D.C. I have not the means of identifying the species ; I inclose, 

 therefore, the wings of a specimen, so that some entomologist 

 may give you the right name.^ The females of Pceciloptera, as 

 indeed of many other Fulgorida?, are known to have the property 

 of secreting a wax-like substance from between their abdominal 

 rings, and especially from pecuhar appendages of the last ring. 

 This substance is, in the present species, of a beautiful white 

 colour, glossy like silk, and formed of exceedingly thin threads, 

 1-500 to 1-700 of a millimetre thick, and generally less than a 

 centimetre long. When taken off the li\-ing insect, the latter will be 

 found to produce new threads in somewhat less than twenty-four 

 hours. The threads are pure wax, lighter than water, insoluble 

 in cold alcohol and ether, but dissolving a Httle in hot alcohol, 

 and very easily in hot benzol. The fuFing-point I found by 

 repeated experiments to be a little higher than that of boiling 

 water, though I could not determine it exactly, owing to the 

 small quantity of wax I had collected (from 150 insects I ob- 

 tained but six centigrammes of wax). In a heated silver spoon, 

 or on platin-foil, the wax melted very easily, leaving no residue 

 whatever. The molten wax was at first of a light yellow coloiu", 

 which disappeared again after its getting cold. 



The late Mr. D. Hanbury, in a paper on the insect-white- 

 wax of China ("Science Papers," 62), quotes the statement of 

 Capt. Hutton as to the. properties of the wax of Plata limbata, 

 an insect closely related to the Pceciloptera. It is said to " dis- 

 solve readily in water, while the attempt to melt it on the fire 

 without water or oil proved altogether abortive, the wax merely 

 burning and consuming away till it became converted into a 

 hard and baked substance." This is certainly very singular, and 

 it appears to me highly questionable whether Capt. Mutton's 

 so-called wax was any wax at all. 



I have not been able to find out what can be the benefit the 

 insects derive from these copious secretions ; but as they occur 

 only in the females, there will probably be some connection with 

 the egg- laying or hatching process. I observed no males, and 

 could discover no eggs even in places where the females were 

 thickly crowded on the branches. I should add that the insect 

 is generally rather uncommon in our local fauna. 



Caracas, July 15 A. Ernst 



Spontaneous Combustion of Wasps' Nests 



Some time ago the house of General P. M. Arismendi 

 (now Consul of Venezuela, in Port-of- Spain, Trinidad) in this 

 city, had a rather narrow escape from being set on fire by the 

 spontaneous combustion of a large wasps' nest (a species of 

 Polistes) in a closet under a roof. The day was exceedingly hot ; 

 but this circumstance, I think, has a very slight connection (if 

 any at all) with the outbreak of smoke from the nest. Roofs 

 in this country are constnicted of tiles supported by a thick 

 layer of compact earth, which rests on the usual lath-work of 

 dry canes (the stems of Gynerium saccharides, or arborescent 

 grass), both being substances that xonduct heat very badly. 



' An entomological correspondent informs us that the wing is that of 

 Pocciloptera phala-noides, Lian., and agrees with Stcll's figure of the ir.sect 

 frjm Surinam. — Ed. 



