May 1, 1895.] 



KNOWLEDGE 



101 



blood, whilst the production of chlorophyll in plants has 

 been experimentally proved to be, in some way as yet 

 imperfpctly understood, dependent on the presence of iron 

 in their nutriment. According to Ehrenberg some species 

 of diatoms secrete ferric oxide in considerable quantities. 



But the existence of iron is not confined to our own 

 planet. The spectroscope reveals its presence in the sun 

 and many of the stars. It is also the chief constituent of 

 meteorites. 



Native iron is of very rare occurrence among the terres- 

 trial rocks. Veins are all but unknown. It has most 

 frequently been detected in the form of grains scattered 

 through certain eruptive rocks, such as the gabbros 

 belonging to the volcanic outbursts of Mull and Skye 

 during the Tertiary period, and in the basalt of the Giant's 

 Causeway. Nordenskiold has discovered in the island of 

 Disco, off the west coast of Greenland, a number of large 

 masses of iron, one weighing nearly twelve tons; but 

 whether they are of terrestrial origin is doubtful. Similar 

 masses occur in the basalt of the vicinity. The great 

 traveller himself regarded them as memorials of a meteoric 

 fall during the ouitlowing of the rock in Tertiary times ; 

 but Daubree has shown that the rock contains microscopic 

 part cles of iron, associated with certain other minerals 

 in such a way as to exclude the hypothesis of the con- 

 junction being accidental. He therefore concludes that 

 the iron came from below with the other constituents of 

 the mass. 



This subject naturally raises the question, so often asked 

 in view of the high density (about 5'5) of the earth as a 

 whole compared with the average density (say 2o) of the 

 surface rocks, viz., whether the interior contains large 

 quantities of iron or other uncombined metals. Taking 

 as a guide Sir A. Geikie's list of the sixteen most abundant 

 elements— to wit. 0, Si, C, S, H. CI, P, F, Al. Ca, Mg, K, 

 Na, Fe, Mn, Ba — it is observable that their heaviest com- 

 binations with one another barely reach the minimum 

 specific gravity required to account for the earth's density. 

 Whether the enormous pressure, vastly greater than any 

 whose effects we can observe in our laboratories, to whicu 

 the earth's internal layers are subjected, would serve to 

 compress the materials to the requisite degree is ex'^eedmgly 

 doubtful, whilst it is certain that the high internal 

 temperature of the earth's interior must, to a large extent, 

 counteract the reduction of volume through pressure. It 

 seems most probable, therefore, that extensive deposits of 

 heavy materials of some kind exist in the interior of the 

 earth, and of such none is more likely to abound than iron, 

 considering its high rank as a constituent of the crust. 



Meteoric iron is known in masses varying from many 

 tons in weight down to microscopic grains. The latter 

 have been detected in the snows of the Alps and the 

 Arctic regions, and caught on board ship in mid-ocean by 

 means of sheets of glass smeared with glycerine and 

 exposed to the wind. Grains of metallic iron abound m 

 the red clay of the Atlantic Ocean, a fact which may be 

 taken as a proof of its slow growth. M>-tejric iron is 

 invarialily alloyed with metallic nickel. Until recen ly 

 the natural occurrence of "nickel-iron" (as the alloy is 

 termed, uot'nithstaiidlng the predominance of the latter 

 elemei t) was unknown except as a constituent of meteorites. 

 Masses of an alloy of the two metals (with oih.-r miterials) 

 have, however, been lately discovered in the gravel of a 

 stream in Oregon, which differ in some remarkable respects 

 from all meteoiites hitherto known. Tnus they do not 

 exhibit the peculiar markings, termed " WiJmannstatt's 

 figures," when treated with nitric or hydrochloric acid. 

 Josephinite is the name which has been given to the new 

 mineral. 



Ii'on is also found alloyed with platinum. A specimen 

 from Siberia analyzed by Berzelius was found to contain 

 86-50 per cent, of platinum, R-32 per cent, of iron, 

 together with small quantities of palladium, rhodium, 

 copper and " gangue." Another sample from South 

 America contained, of platinum 84 30 per cent., of iron 

 5-31 per cent., of rhodium 3-46 per cent., besides palladium, 

 iridium, osmium and copper, seven metals in all. 



BREATH-FIGURES. 



By Dit. .J. G. McPhersov, F.R.S.E., formerly 

 Mathematical Examiner in, tlie Unieersity of St. Andrews. 



THERE is something exceedingly fascinatmg about 

 the curious set of phenomena known as breath- 

 tigures, and the explanation of their existence. 

 New light has lately been thrown upon their 

 nature ; and their study is interesting. 



Fifty years ago. Prof. Karsten, of Berlin, placed a coin 

 on a piece of clean plain glass, and passed through it a 

 current of electricity. Nothing was seen on the glass when 

 the coin was removed, but when he breathed on the plate 

 the characters of the coin became visible. At the same 

 time Sir W. E. Grove succeeded in producing impressions 

 with simple paper forms. Mo-er, of Konigsberg, produced 

 figures on polished surfaces by placing on them rough 

 bodies. Riess described a breath-track made on glass by 

 a feeble electrical discharge. 



But Mr. W. B. Croft has lately been investigating the 

 matter with exemplary care and perseverance; for it requires 

 some pract ce to manage the electrification properly. Tiiig 

 was his most successful plan : Place a glass plate on a 

 table for insulation, and pat a coin of any metal on the 

 centre of the plate. In many cases the ima^'e on the coin 

 does not touch the glass on a'^count of the pr jecting ring ; 

 but these seem to be best suited for the expeiiment. 

 Arrange a strip of tinfoil from the coin to the edge of the 

 glass ; on the coin place a smaller plate of glass, and above 

 that plate place a second coin. Connect the tinfoil and 

 the upper coin With the pjles of an elejtric mach.ue, and 

 turn th^ handle of the midline for two minuter, so thai 

 continuous spirks miy piss. O.i tiki^g up the glass, 

 nothing can be seen on it, even with the help of a magni- 

 fying glass. Yet on the glass there is a latent impression ; 

 for, by breathing on the side of the glass next the coin, a 

 clear frosted picture of that side of the coin which had 

 faced it will be produced, even to the smallest details. The 

 whole projecting parts of the coin have a black counterpart, 

 and there is a marvellously fine gradation of shade 

 corresponding with the depth of cutting on the coin. If 

 this breath-figure be exammed under a microscope, the 

 moisture will be seen really deposited over the whole ; but 

 the size of the minute water-particles increases as the part 

 of the piiiture is darker in slude. Arouad the coin's disc 

 is a black ring, a qmrter of an inch in b -eaJch. Shjuld 

 ilie coin used have milled eJges, radidl lines will pass 

 through this ring. 



If ihese breaih-figares are carefully projected, th-re is 

 no apt)arent limit to tneir permiuerice, even far ye.irs. 

 Alon Us alter they ha^'e l)eeii s=;t a-i le, the bltck liug 

 roand the disc graduiUy cliau^'es mto sev.;ral rings, 

 forming beauiifal concentric al Kruatious of bUck and 

 while. If hall-a-.d izeu coins, lying lu c .niact side by side 

 in the form of a cross, be placed oa insulated glass, then 

 over the coins a test glass, with a corresponding cross of 

 cokis above it, beautiful breath-figures will be produced. 

 In the black spaces between the circles are clear white lines 

 which are common tangents to the circles, when the coin.s 



