220 



NATURE 



Sjnly 8, 1880 



Dilatation of the Iris 



In addieion to the method of ob.-ervation mentioned by Mr. 

 Ackroyd in his photometric proposal (Nature, vol. x.\i. p. 627) 

 I may mention that the variations of the diameter of the pupil 

 are very beautifully observed by a pair of punctures in a screen 

 over the eye. In fact long ago I used this as a means of observing 

 the absolute diameter of the pupil, subject to a small imexamined 

 constant error. 



By pricking a row of hole? in a card at distances of "06, '07, 

 •oS .... "25 in;h, and placing this close over the eye, the 

 diameter is observed by sliding the card until two of the holes 

 are found at such a distance that their edges appear to touch. 

 Tlie opening of the other eye, or the slightest disturbance of 

 light, produces an apparent alteration in the sizes of the disks of 

 light, so that their edges recede or overlap ; and a fresh pair of 

 holes may be found showing the altered diameter of the pupil. 



Thus (l) the extreme diameters of the pupil were found to be 

 ■07 (? "06) and '25 inch ; (2) the diameter is rapidly variable at 

 will, -cvUhout any alteration of stimulus, even as much as from 

 •13 to '19 inch ; (3) the sympathetic motion takes "4 or '5 second 

 to be produced when the other eye is exposed to light. Many 

 .similar interesting questions may be examined by this simple 

 instrument ; for which purpose I inclose a sample card for 

 editorial trial. W. M. F. P. 



Comparative Curves in Terrestrial Magnetism 

 Monsieur, — En sequence a la interessante communication de 

 Rev. S. J. Perry de I'Observatoire de Stonyhurst, je vous prie 

 d'additioner les suivanls renseignements surla meme perturbation 

 magnetique de 17 mars passe, d'aprcs la coin-be dit declinographe 

 de I'Observatoire dn Infant D. Luiz, a Lisbonne. 



La difference entre le maximum et le premier minimum a 

 5h, 49,5m. G.M.T. est de I3',9 (un tiers du mouvement a 

 Stonyhurst), et entre le mC-me maximum et le 2" minimum a 



Declinographe 17 Mars, iSSo (Lisbonne M.T.). 



loh. 45m. G.M.T. est un peu plus grancle l6',2, le contraire 

 qu'on voit a Vienne et Stonyhurst. 



II est digne aussi de remarque que le temps du 1"=' minimum ne 

 s'accorde avec le temps a Stonyhiu-st et Vienne, pendant que le 

 temps du second est de parfait accord. 



La longitude de cet Observatoire e.-t + 36m. 35s. G. Je vous 

 envoie la copie de la courbe. 



Agreez, Monsieur, I'assarance dc ma haute consideration. 



Lisbonne, 21 jnin iSSo J. Capello 



Effects of Lightning on Trees 

 Your note in Nature, vol. xxii. p. 204, on the recent 

 thunderstorm at Geneva induces me to send you a note on a tree 

 struck by lightning in Stoneleigh Park during a severe storm on 

 last Thursday week (June 24). The tree was a fine oak about 

 forty feet high, and the lightning seemed to have struck not among 

 the smaller branches at the top, but about two-thirds of the way 

 up the main trunk, just where several of the larger branches 

 came off from the stem. From this point to the ground the bark 

 had been rent off along a strip about three inches wide, and 

 through the whole length the wood beneath the bark had been 

 gauged out as if by a carpenter's tool, the groove made being 

 about an inch wide and deep. The curious fact of the tree being 

 struck apparently among the branches at once suggested to me 

 that the electricity must have travelled, without visible effect, 

 through the upper branches, and only produced disruption of the 

 wood when the current was strengthened by the combination of 

 a great number of separate streams. I had forgotten that this 

 was Prof. Colladon's theory of electric discharge, but am glad to 

 be able to give it the support of this observation. 



Rugby, July 3 L. Cumming 



Iron and Hydrogen 

 In the description given a few weeks back of the experiments 

 of Prof. Hughes, the fact was deraon.itraled that iron wire in 



contact with dilute acids becomes brittle, and at the same time 

 takes up hydrogen. 



There are one or two points of great interest that many, 

 perhaps, besides myself, would like to know more about. 



Thus, at the same time the iron becomes brittle, does it also 

 become harder? 



Tliis leads one to speculate on the facts illustrated in the 

 hardening and tempering processes of steel. 



We know that such liquids as water, weak acid, oil, &c., 

 which are used as baths in which the heated metal is quenched, 

 are all decomposable by iron and other metals at a high tempera- 

 ture, the result being the liberation of hydrogen, &c. Now 

 is it not probable that this liberation of hydrogen is really the 

 essential element in the physical change produced in the hardened 

 steel ? — that is to say, that the steel absorbs, or perhaps becomes 

 alloyed with the nascent hydrogen in contact with its surface, 

 thus rendering it intensely hard? 



Prof. Hughes has pointed out that a red heat entirely dissi- 

 pates the hydrogen from the iron wire, which returns to its 

 normal state. 



This perhaps will explain the process of tempering by 

 supposing that a certain propartion of the (hardness-rendering) 

 hydrogen is driven off according to the temperature reached, 

 as shown in the well-known shades of colour seen on the surface 

 — that is to say, the hardness is proportionate to the contained 

 hydrogen, such as that many other metals become very hard or soft 

 by being alloyed as zinc and copper in brass, tin and copper in 

 bronze, &c. 



This is supported by the fact that one of the most successful 

 processes of hardening depends on the use of a quenching-bath 

 of dilute sul]>huric acid. This would be explained by the 

 greater ease with which acidulated water is dec imposed by iron, 

 and therefore a larger bulk of the nascent hydrogen liberated on 

 its surface could be absorbed by the metal. 



The carbon in steel probably only plays the part of a go- 

 between in rendering the absorption of hydrogen more facile. 

 Tliere is a fact that also supports this, namely, if unhardened 

 steel is dissolved in HCl the carbon is left in the form of 

 graphite scales, whereas after hardening, if treated with the 

 acid in the same manner, the residue is found to consist of a 

 liquid hydrocarbon, thus showing the presence of hydrogen in 

 the metal. 



These points I should like to have been able to confirm or 

 refute for myself ; but not having the required time or apparatus, 

 I leave it with the hope that some one possessing those advan- 

 tages will settle these questions. H. J. Johnston-Lavis 



Naples 



" Coronella laevis" 



In Nature, vol. xxii. p. 156, the presentation is announced 

 of two specimens of Corondla lavis (British) to the Zoological 

 .Society. I have known so many pei-sons doubt the existence of 

 the Coronella in the New Forest that I should feel greatly 

 obliged to any of your correspondents who would give me some 

 information as to its history, whether it has been introduced, or 

 is really indigenous. H. King 



[Mr. Sclater tells us that he has no doubt that the smooth 

 snake is indigenous to the British Islands, although it was over- 

 looked for many years. The first living example received by the 

 Zoological Society was in August, 1862, presented by Mr. 

 Fenton, having been obtained in the neighbourhood of Sand- 

 hurst. Since then nearly twenty specimens have been received, 

 chiefly from the New Forest and neighbourhood of Bournemouth. 

 See Mr. Cooke's excellent little volume, "Our Reptiles" 

 (London, 1S65), for a full account of this species of snake. 

 —Ed.] 



Recall of Appearance of Books, &c. 



I HAVE only to-day been able to read the back numbers of 

 Nature for the past two or three months, and hence have only 

 now seen Mr. Ernst's letter in your issue of April 29 last. 



His power of recalling the appearance of books I know is 

 posse^sed by others. I have a very large and still increasing 

 library, but there is hardly a volume, or indeed a tract, the 

 appearance and condition of which does not at once present 

 itself to my mind if occasion to use it should arise. Further, 

 being engaged in the compilation of a work some years smce, 

 wherein many references to other books were necessary, I used. 



