Feb. 24, 1888.] 



SCIENTIFIC NEWAS. 



185 



layers was to steep the stones in honey or in oil, whilst 

 they are repeatedly boiled and finally raised to a heat 

 sufficient to char the organic matter. In consequence 

 the honey, etc., is decomposed, and its carbon is deposited 

 in fine particles in the pores of the stone. If any portion 

 is more absorptive than the rest it takes a black or deep 

 brown colour, whilst the impervious portions remain 

 white or colourless. Such is the origin of the black and 

 white banded onyx from which cameos are cut. Instead 

 of applying heat, the same result is obtained more 

 conveniently by first steeping the stones in honey and 

 then treating them with strong sulphuric acid. Thus the 

 honey was decomposed and its carbon was deposited in the 

 pores of the stone just as if a strong heat had been applied. 



Chalcedony may be very easily coloured blue, entirely 

 if its texture be uniform, or in stripes if it consists of 

 layers more or less pervious. The stones are steeped in 

 potassium ferro-cyanide, better known by its old name of 

 yellow prussiate of potash, and after becoming thoroughly 

 saturated are transferred to a solution of per-nitrate ol 

 iron. Or instead, they may be steeped first in potassium 

 ferri-cyanide (red prussiate of potash), and then placed 

 in a solution of copperas, otherwise known as green 

 vitriol, or ferrous sulphate. 



A richer blue, more inclining to a violet, is produced 

 by soaking the stones first in a solution of copper nitrate 

 and then in ammonia. 



The delicate green of chrysophrase may be reproduced 

 by laying transparent chalcedony in a solution of nickel 

 nitrate, 'preferably such as contains a little nitrate of 

 cobalt. The action is slow, and takes several weeks. 



Or, instead of nitrate of nickel, a nitrate of chromium 

 may be used. 



It is perfectly obvious that on similar principles 

 chalcedony may be coloured a pink or a rose shade by 

 steeping it in a solution of nitrate of cobalt. 



The red cornelian can scarcely be said to occur as 

 such in nature. In some districts, especially at Cambay 

 in India, yellow chalcedonies of very good quality are 

 found. If these are exposed to the sun for two years 

 the hydrated ferric oxide which they naturally contain is 

 deprived of its water, and the colour becomes a rich red. 

 Or colourless chalcedony is steeped in a solution of 

 nitrate of iron, and is then either exposed to the sun or 

 to artificial heat. 



It need scarcely be said that various coal-tar colours 

 have been used for the artificial colouration of stones, 

 but with very scanty success. It is, indeed, exceedingly 

 easy to introduce the colours into the pores of the stone, 

 but their want of permanence on prolonged exposure to 

 light has proved a fatal obstacle. 



The art of stone-colouring, old as it is, admits of great 

 development, especially in its application not to personal 

 decorations, but to the purposes of the architect. 



— •>-;»«^>^fe^ — - 



A ONE-COLOURED RAINBOW. 



A T the meeting of the Royal Society of Edinburgh on 

 ■^"^ the 30th January a communication of great interest 

 from Mr. John Aitken was read. He described a mono- 

 chromatic rainbow which he had seen on Christmas Day 

 when walking on the upper ground near Falkirk. 

 To ordinary minds the idea of a one-coloured rainbow is 

 paradoxical. From the time when its colours were first 

 laid down till now we have not noticed the usual seven 

 coloured phenomenon altered very much. Yet here Mr. 



Aitken assures us that he saw a red rainbow ; and we are 

 bound to accept the veracity of so accurate a physicist. 



In the Eastern sky he observed a strange pillar-like 

 cloud illumined by the setting sun ; and instead of the 

 glow being horizontal and rising from the horizon, it was 

 vertical. Soon the illumination became more elevated ; 

 and he began to suspect that this was not a cloud at 

 all, but the "tootle" of a rainbow. Gradually the red 

 pillar stretched out, turned over, and became a complete 

 arch bespanning the north-eastern heavens. He never 

 saw anything like it before ; for there was no colour in 

 it but red. There before him was the ruddy arch, like 

 a mysterious bow of blood, quite uniform. All the other 

 colours were concealed by the strange red hue. After a 

 time, on careful inspection, he observed at one or two 

 points traces of yellow ; but of the others there was not 

 a vestige. In their place, however, he observed a dark 

 band stretching inwards to about the usual breadth of 

 those colours in the rainbow. The band was distinctly 

 darker than the sky within the bow ; but not greatly so. 

 Outside of the rainbow he observed part of a secondary 

 bow ; and at several places there were traces of a super- 

 numerary bow ; the red showing itself at different places 

 in the inner edge of the 'dark band. 



What was the cause of this strange phenomenon of a 

 one-coloured rainbow ? Mr. Aitken took a careful look of 

 all the scenery round and above him. The Ochil Hills, 

 which lie to the north of where he was standing, were 

 then covered with snow, and the setting sun illumined 

 the snow-clad hills with rosy light. Never had he seen 

 such a depth of colour in the fiery glow, even in the- 

 Alps ; the appearance was more of a furnacy red. There 

 vi'as a strange coincidence in the ruddy rainbow, and the 

 deep red colour of the snow-clad hills. Knowing that the 

 rainbow is simply nature's .spectrum analysis ot the 

 sun's light, these phenomena showed to Mr. Aitken that 

 the sun's light was shorn of all the rays of short wave 

 lengths on its passage through the atmosphere, only 

 the red rays reaching the surface of the earth. 



It occurred to him that every object on that afternoon 

 ought to have appeared red, and nothing but red, if the 

 theory were true that the red rays alone reached the 

 earth's surface. Yet that was not the case. Everything 

 looked to him almost the same as ordinary except for 

 the glare of the hills and the reflection of the bow. 

 Things were simply tinted with red, as we have seen 

 in a brilliant afterglow. And he accounted for this by 

 the fact that, while we only get red light direct from the 

 sun, there was a great deal of green, blue and violet 

 light reflected from the sky overhead ; the combination 

 of these with the red caused the light to be but little 

 different from the ordinary. A dense curtain of clouds 

 was overhanging the hills, and screening off the light 

 of the sky ; this prevented the other colours from com- 

 bining with the red which was thus left on the snow of 

 the Ochils and the cloud in the eastern heavens. 



Cast Iron Columns in Fires. — From experiments made 

 recently in Berlin upon the behaviour of columns of wrought 

 and cast iron, granite, marble, sandstone, limestone, and 

 concrete, when subjected to the conditions which obtain in 

 fires, it appears that cast iron stands the action of fire and 

 water best, as it continues to support the load even when red- 

 hot and cracked in places, whereas wrought iron gives way 

 easily. Concrete proved to be better than marble, granite, 

 limestone, sandstone, and ordinary brickwork. None of the 

 natural stones resisted the fire, though granite was the best. 



