68 



NA rURE 



{May 23, 1872 



tion of stannous chloride, it is quickly surrounded with an 

 outgrowth of prolonged octohedra, and as these advance 

 into the liquid it is easy to observe that the additions of 

 new metal commence at the apex, and that the wave of 

 chemical change proceeds down the lateral edge, occupy- 

 ing some seconds of time in depositing the new layer of 

 material. Frequently, also, there is a luxuriant growth of 

 large Hat leaflets, or of symmetrical structures resembling 

 fern-leaves, but with the fronds arranged at right angles, 

 or combinations of these with octohedra, as shown in 

 Fig. 6. These fern-leaves often begin of a dull grey 

 colour ; but, as they advance, suddenly change to a bril- 

 liant Avhite. 



The paiticular form of these crystalline growths de- 

 pends, therefore, primarily, on the specific character of 

 the metal ; but this is greatly modified by the strength of 

 the solution. 



The forms assumed by native metals resemble those 

 produced by this process of substitution. In some cases, 

 indeed, it seems almost certain that the deposition of 

 these minerals was effected in the same way, as, for in- 

 stance, the silver which occurs sometimes in tufts, some- 

 times in large crystals, on the native copper of the Lake 

 Superior district. Gold is frequently found in cubes more 

 or less rolled, but the leaf gold from Transylvania bears 

 a striking likeness to the crystals that form in our labora- 

 tory experiments. Silver is often found native as twisted 

 hairs or wires of metal — a form that never occurs in the 

 decomposition of its nitrate by copper, but which can be 

 artificially produced in another way. 



There has been noticed a singular tendency in old 

 silver ornaments and coins to become cr\stalline and 

 friable. Here is an ancient fibula from the island of 

 Cyprus, supposed to be at least 1,500 years old, which, 

 through the greater portion of its substance, presents a 

 fracture something like that of cast-iron, and us specific 

 gravity has been reduced in round numbers from 10 to 9. 

 It contains a little copper. This property of certain 

 metals or their alloys to change in condition and volume, 

 is worthy the attention of those whose duty it is to make 

 our standards. Experiments should be instituted for the 

 purpose of learning what metals or comljinations of metals 

 are least subject to this secular ch.inge. 



These meiallic crystals are Nature's first atteiript at 

 building. This material is the simplest possible — in fact, 

 what chemists look upon as elementary. But how is the 

 building carried on.'' What are the tools employed.' 

 Where are the bearers of burdens that bring the prepared 

 pieces and lay them together according to the plan ol the 

 Great Architect .' We must try to imagine what is taking 

 place in the transparent solution. The silver, of course, 

 existed at first in combination with the nitric clement, and 

 for every particle of silver deposited on the growing tree, 

 an equivalent particle of copper is dissolved from the 

 surface of the plate. The nitric element never ceases 

 to be in combination with a metal, but is trans- 

 ferred from the one metal to the other. On the " Polari- 

 sation Theory,' the positive and negative elements of the 

 salt constantly change places and enter into fresh combi- 

 nations, one consequence of which would be a gradual 

 passage of the nitric element from the growing silver to 

 the copper plate. This actually takes place, and there is a 

 diminution of the salt at the ends of the silver branches, 

 giving rise to an upward current, and a condensation of 

 nitrate of copper against the copper plate, which gives rise 

 to a strong downward current. These two currents are 

 seen in every reaction of this nature. In the case of silver 

 and copper, however, it has been proved that the crowd- 

 ing of the salt towards the copperplate is more rapid thai, 

 would follow from the usual polarisation theory. Tne in- 

 strument employed for determining this point was a 

 divided cell in which two plates, one of silver and the other 

 of copper, connected together by a wire, are immersed 

 e^gh in a solution of its own nitrate, contained in each 



division of the cell, and separated from one another 

 merely by parchment paper. The crystals of silver de- 

 posited on the silver plate in this experiment are very 

 brilliant. 



There arc other indications of the liquid being put into 

 a special condition by the presence of the two metals which 

 touch one another. Thus zinc alone is incapable of de- 

 composing pure water ; but if copper or platinum be 

 deposited on the zinc in such a manner that the water can 

 have free access to the junction of the two metals, a de- 

 composition is effected ; oxide of zinc is formed, and 

 hydrogen gas is evolved. At the ordinary temperature the 

 bubbles of gas rise slowly through the liquid, but if the 

 whole be placed in a flask and heated pure hydrogen is 

 given off in large quantity. We have also found that iron 

 or lead similarly brought into intimate union with a more 

 electro-negative metal, and well washed, will decompose 

 pure water. 



As might be expected, the action of magnesium on water 

 may be greatly enhanced by this method ; and a pretty 

 and instructive experiment may be made by placing a coil 

 of magnesium in pure water at the ordinary temperature, 

 when there will be scarcely any eft'ect visible, and then add- 

 ing a solution of sulphate of copper. The magnesium is 

 instantly covered with a growth of the other metal, and at 

 the same time the liquid seems to boil with the rapid 

 evolution of hydrogen bubbles from the decomposed 

 water. 



When, however, the force of the two metals in contact 

 has to traverse a layer of water, the resistance offered by 

 the fluid prevents its decomposition. This must also be 

 an important element in the decomposition of a metallic 

 salt dissolved in water, and in fact we have found that the 

 addition of some neutral salt, such as nitrate of potassium, 

 increases the action — apparently by diminishing the resist- 

 ance of the liquid. If, too, wc increase the quantity of 

 the dissolved metallic salt, we get more than a propor- 

 tional increase of deposited me;al. Thus, in an experi- 

 ment made with the different strengths of nitrate of silver 

 on the table, the following results were obtained in ten 

 minutes, all the circumstances being the same except the 

 strength of the solution : — 



1 per cent, solution dissolved "025 grm. copper. 



2 „ „ „ -078 „ 



4 V. „ „ '224 



In fact it has been found that in solutions not exceeding 

 5 per cent., twice the amount of nitrate of silver dissolved 

 in water gives three times the amount of chemical action; 

 and this is true with other metals also in weak solution. 

 It may be that this is not the precise expression of a phy- 

 sical law, but it agrees at least very closely with the results 

 of experiment. 



The power arising from this action of two metals on a 

 binary liquid may be carried to a distance and produce 

 similar decompositions there. This is ordinary electrolysis. 

 Metals have often been crystallised from their solutions in 

 this way, and Mr. Braham has made excellent preparations 

 of crystalline silver, gold, copper, tin, platinum, &c., by 

 using poles of the same metal as that intended to be 

 deposited upon them. The forms thus obtained are pre- 

 cisely analogous to those produced by the simple immer- 

 sion of one metal into the soluble salt of another, and 

 illustrate still further the essential unity of the force that 

 originates the two classes of phenomena. 



NOTES 



The speech of the Prime Minister at the meeting held last 

 week in support of the fund to pay olT the debt at King's 

 College was a striking comment on some recent utterances of 

 members of the Government to the effect that Science is well 

 able to support itself, and needs no assistance from the State. 

 The claims of this fund on public assistance were earnestly and 



