602 



NA TURE 



[April 22, 1880 



London to gravity at the equator would eventually be known, 

 and meanwhile a base of connection was wanted. It is perfectly 

 true a; I have already said, that an absolute determinination is 

 eminently satisfactory, and (theoretically) can stand by itself ; 

 but practically they rarely did so. It is perfectly true that if the 

 length of a pendulum is actually measured and its rate observed, 

 an independent determination is made ; but practically the 

 determination was almost always relative. The pendulum « as 

 generally not so much measured as to its actual length, whatever 

 that might be, as adjusted to a certain length such as (very 

 commonly) had been previously done at Paris or London. The 

 distinction is very clear in some cases, less so in others. But, 

 generally speaking, the determination has as good a right to be 

 classed among the differential ones as among the abs >lute. 



Consider the case of Graham's pendulum as used by Campbell 

 at Jamaica. It was purposely designed to be adjusted to the 

 same length. Or, again, consider Legentil's. He was constantly 

 testing and adjusting the length by means of a regit enjer, and 

 the only kind of measurement which took place was that of 

 examining the equality from time to time of the length of his 

 pile fibre. 



It appears to me to be entirely beside the mark to insist that 

 his regie or gauge had been compared or measured. It was used 

 as a gauge and not as a measuring scale. 



The same applies in nearly ad cases. A gauge is always 

 found to have been used, and some constant addition or subtrac- 

 tion made for the calculated position of the centre of oscillation. 



That which gives to all the older determinations their appa- 

 lently absolute character is that the result is expressed in linear 

 measure. Considering the exceedingly doubtful character of the 

 linear element so introduced, it is practically certain that the 

 only chance of utilising any of these is to get back to the 

 observed rate if possible, and to treat them all as merely 

 differential. 



Let it not be supposed that we shall lose anything by this. As 

 things now stand, observations which were essentially differential 

 and often good of their kind are under the cloud of doubtful 

 reduction, caused by the endeavour to kill two birds with one 

 stone. Experience has show n that this is barely possible even 

 now, with vastly better means Common stnse sugge ts that it 

 was vain before. 



I have hitherto been speaking of the last century. The aspect 

 changes somewhat as we enter the present one. Scarcely a trace 

 remains of the absolute force of gravity as a real object. The 

 idea of a linear standard is still active, but evidently doomed. 

 What will be left as the motive for absolute determination, in 

 preference to differential ? I confess that I can give no answer. 

 Anxious as I have been, and am, to learn and to understand 

 the whole of this subject ; careful as I may be to catch at every 

 indication of an unexpressed idea latent in the mind ; it is in 

 vain that I try to find a raison d'etre for absolute pendulum 

 operations at the present day. It would be impossible to say 

 this and not imply dissent from the views of those who advocate 

 then- prosecution, and I am well aware that such views are advo- 

 cated by a section of the Continental geodesists. But I seem to 

 be unable otherwise to find a solution. A year has elapsed since 

 this paper was written — all but these two sentences — and I have 

 learnt nothing to change my opinipn. J. HeRSCHEl 



NOTE ON SOME EFFECTS PRODUCED BY 

 THE IMMERSION OF STEEL AND IRON 

 WIRES IN ACIDULATED WATER ' 



DURING a discussion upon a very interesting paper by our 

 president, "On the Durability of some Iron Wire," I men- 

 tioned a fact which I had lately observed, viz., that steel or iron 

 wires immersed for a few minutes in acidulated water containing 

 one tenth sulphuric acid became excessively brittle. Our president 

 has since kindly asked me to make a few more experiments on 

 this subject, and to embody them in the form of the present 

 note. 



Upon repetition of these experiments I have found that this 

 brittleness is no mere accidental result, due to some flaw in the 

 steel or iron wires, but that the resulting brittleness is invariable 

 in all kinds of steel as well as iron. Nor is the effect due to any 

 specific proportions of sulphuric acid to the water ; nor, in fact, 

 as we shall see later, to any particular acid. The effects, how- 

 ever, seem confined to steel and iron ; as by similar treatment 



1 Read before the Society of Telegraph Engineers, April 14. by P r °f' 

 D. E. Hughes. 



I have as yet obtained no perceptible effect on copper or brass. 

 At first I was inclined to believe that the effects were due 

 primarily to a change in the molecular structure ; but a more 

 extended series of experiments has led me to adopt entirely the 

 view taken by my friend Mr. W. Chandler Roberts, who pre- 

 dicted that the effects were most probably due to the absorption 

 of hydrogen. 



I have tested these wires in my induction balance, but can 

 find no change whatever in its magnetic conductivity, nor any 

 change which would be the equivalent of those produced by 

 heat, strain, torsion, or tempering; but there are very evident 

 results produced : if the conditions of the experiments are such 

 as to favour the absorption of hydrogen. For instance, if we 

 reduce the proportion of sulphuric acid to one-twentieth, we find 

 that it requires some thirty minutes' immersion to prcduce the 

 full effect, a few minutes' immersion producing no perceptible 

 result. If now we place an amalgamated zinc plate in the same 

 liquid, and join the two extremities, we have an ordinary 

 1 attery, where hydrogen is given off on the steel wire. Now 

 as the hydrogen produced by the decomprsition of the water 

 is much more rapid than before, we find that a few minutes' 

 immersion produces a far more brittle wire than could be ob- 

 tained by hours of simple immersion, and we have the result 

 free from any doubt as to its being a mere surface action ; for it 

 we immerse the wire alone, surface corrosion rapidly takes place, 

 but by simply connecting it with the zinc the steel is perfectly 

 protected, retaining its original bright surface, for any time, as 

 long as it is so protected. 



It is not absolutely necessary that we should join the zinc in 

 the same cell, for if we pass a current from a few cells of an 

 external battery through two steel wires as electrodes in sulphuric 

 acid and water we find that both wires have became brittle, 

 though in a very different degree, the wire connected with the 

 zinc or negative pole remaining bright, although excessively 

 brittle, whilst the one connected with the positive pole is much 

 corroded, and but feebly brittle, with this arrangement. I find 

 that sulphuric acid is no longer required, but that all acids, 

 neutral salts, and ordinary water produce an active effect, the 

 time required being simply as the conductivity of the liquids 

 employed. When water or most neutral salts are used, we find 

 the negative pole quite bright, but brittle, the positive pole much 

 corroded, but not at all changed as regards its flexibility. 



I believe that these effects are due to the absorption of 

 hydrogen when the hydrogen is in the " nascent " state, for I 

 have obtained no results by continued immer.-ion in carburetted 

 hydrogen gas (ordinary lighting gas), but when plunged into a 

 medium containing the hydrogen just freed from its combination, 

 its effects are most remarkable : for if we immerse a wire into 

 sulphuric acid and water, say one-twentieth, the effects are slow, 

 requiring at least thirty minutes ; but if we let fall into this water 

 some scraps of zinc hydrogen is rapidly given out, and by now 

 immersing the steel wire in this gaseous liquid, taking care not 

 to touch the zinc, we find that the steel becomes rapidly brittle, 

 whilst its surface is free from corrosion, due no doubt to the 

 protecting surface of surrounding hydrogen. 



Hydrogen seems to permeate through the entire mass, for 

 iron rods a quarter of an inch thick were equally affected, re- 

 quiring more time, or in other words, a supply of nascent 

 hydrogen sufficient for the larger mass ; and once the wire has 

 become hydrogenised (if we may be allowed the expression), 

 it retains it under all circumstances of time and change of 

 surrounding atmosphere : heat alone, of all the means I have 

 tried, has any effect ; and if we heat a wire to cherry red in a 

 spirit lamp we find that it is completely restored to its primitive 

 flexibility in a few seconds. This same wire, however, on being 

 immersed in the accidulated water, rapidly becomes again brittle ; 

 we may thus at will render the same wire flexible by previously 

 heating it, or render it exceedingly brittle by favouring its 

 absorption of hydrogen. 



I have remarked that a wire immersed in sulphuric acid and 

 water of any proportion, say one-sixteenth, becomes more electro- 

 negative than at the first instant of plunging. If we take 

 amalgamated zinc as the positive element, and a steel or iron 

 rod or wire for negative, we find that there is such a remark- 

 able 'similarity of electromotive force between all kinds of steel 

 and iron that we are forced to the conclusion that we are 

 simply testing the electro-negative qualities of hydrogenised iron ; 

 the force being with amalgamated zinc '56. 



I noted here a remarkable fact, and which does not agree 

 with the remits of many authorities. I found that as soon as the 



