Nov. 30, 1888.] 



SCIENTIFIC NEWS. 



565 



cated some very valuable experiments by which its 

 nature might be illustrated. He showed that if one took 

 a small portion of the substance and put it into a liquid 

 which was coloured red or blue by some colouring 

 matter, this colouring matter would permeate the 

 mass, which would be coloured throughout by the liquid 

 passing into it. He showed that it could be coloured in 

 another way. If a piece of the substance be wrapped 

 up in paper, and the paper set fire to, smoke was pro- 

 duced, and this smoke would enter in through the pores of 

 the substance and blacken the whole of the interior of the 

 tabasheer. He would show them how that absorption 

 took place in the case of tabasheer, but before doing so he 

 would point out that there were many other forms of 

 colloidal silica which exhibited some porosity, though to a 

 less extent. There was a curious substance called 

 hydrophane, that appeared dull and opaque when looked 

 at under ordinary conditions, but if it were dropped into 

 water it became almost perfectly transparent, and some- 

 times glowed with all the beautiful colours of the opal. 

 When it was taken out of the water it lost that wonder- 

 ful property, and became dull and opaque. Lately a variety 

 of this substance, hydrophane, had been made use of in the 

 United States ; it was found in Colorado, and was used by 

 our American cousins for making " magic" rings contain- 

 ing photographs ; a small plate of the hydrophane is placed 

 over a photograph. Under ordinary conditions the white 

 opaque substance concealed the photograph, but if it be 

 put into water the waterwas absorbed, and the whole mass 

 would become transparent. They would have to consider 

 why the substance became transparent when it absorbed 

 water. One had a familiar example of it in a piece of 

 blotting-paper, which appeared quite opaque when dry, 

 but it became more transparent when it was wet. Now 

 this hydrophane acted in just the same way. The curious 

 property that colloidal silica had of absorbing liquids had 

 long been known, and made use of in the artificial 

 colouring of agates and similar substances. They would 

 learn presently that those agates were made of 

 bands of colloidal silica alternating with others of 

 crystalline silica. Crystalline silica did not absorb 

 liquid, but colloidal silica did. Certain coloured liquids 

 could be introduced into that portion of the agates 

 which consisted of colloidal silicate. There was a very 

 great demand for the black variety of agate, which con- 

 stituted what was known as onyx. Mourning studs and 

 similar articles of jewellery were formed of that material. 

 Perfectly normal black examples 01 this substance 

 were found in nature, but not in anything like the 

 quantities that were required for the purpose of making 

 this mourning jewellery, and so at Oberstein the people 

 had learned to colour those substances artificially, taking 

 advantage of the porosity of colloidal silica. He would 

 try and illustrate how that was done. The workmen 

 took a stone which might be of a dull grey tint, with no 

 beauty about it at all, and put it into a substance like a 

 solution of sugar or honey, keeping it at a gentle heat 

 by placing it upon a stove. It was found that three 

 weeks was often required for such a substance to 

 pass into the pores of the dull-looking agate. When 

 the sugar or honey has passed into the interior another 

 substance is introduced— oil of vitriol or sulphuric 

 acid — which penetrates in after the sugar and pro- 

 duces a wonderful change. When a solution of sugar 

 was treated with sulphuric acid the water was taken 

 away from the sugar and the black carbon was left 

 behind. 



Now, he thought he had brought forward sufficient facts 

 to illustrate that colloidal silica which existed in agates 

 was moreorless porous in its structure. Hemust point out 

 that there was a very interesting quality of colloidal silica, 

 whichwas probably accounted for by the fact that it con- 

 tained a great number of little pores. Colloidal silica pos- 

 sessed in a remarkable manner the property known as 

 opalescence. If one looked at a mass of colloidal silica it 

 would be seen that it had a milky-blue tint when the light 

 was on it, but if the light be allowed to pass through it, it 

 has more or less of a yellow tint. That peculiarity, that 

 the object looked blue by reflected light, while it appeared 

 yellow by transmitted light, was known as opales- 

 cence; and one of the most wonderful examples of this was 

 the air itself. In the daytime the atmosphere, throwing 

 back the light of the sun, appeared blue, and thus 

 they saw the blue sky ; but when the sun gradually 

 declined in the heavens and passed towards the west 

 the light passed through great thicknesses of atmosphere, 

 and the sky then appeared first yellow, and then red, and 

 all the beautiful colours of the sunset were displayed. He 

 must not follow out the question which had been suggested 

 by Dr. Tyndall and other physicists, who had shown that 

 this opalescence of the atmosphere and all other forms of 

 opalescence were due to minute foreign particles scat- 

 tered through the atmosphere or through the liquid 

 which exhibits the opalescence. A simple experiment 

 would, perhaps, render that tolerably clear. He had a 

 vessel of water, and in the bottle there was a quantity of 

 resin dissolved in spirit. That substance remained dis- 

 solved in the spirit, but it would not dissolve in water. 

 When introduced into the water it was broken up into a 

 countless number of very minute particles, those 

 particles acting upon the light in such a manner as pro- 

 duced opalescence. That quality had received the name of 

 opalescence because it was exhibited by the special form 

 of colloidal silica which was known as opal. 



Both the colloidal forms and the crystalline forms pre- 

 sented very beautiful colours. Those colours were, like 

 the opalescence appeared to be, in all cases due to the 

 presence of very minute foreign substances. Sometimes 

 quartz presented a beautiful greenish tint, at another time 

 an amethyst or purple tint, at other times a bright 

 yellow colour, and at other times it becomes dark and 

 almost perfectly black. But he would strongly recom- 

 mend them to examine the wonderful varieties presented 

 in that substance in the horse-shoe case in that museum. 

 He hoped that after some of these lectures they 

 would take the opportunity of seeing the wonderful 

 beauty and variety of the forms of silica which were 

 there exhibited. How small a proportion of foreign sub- 

 stance might produce a large amount of colour was illus- 

 trated by two vessels on the table. In one he had placed 

 a pennyworth of gold, and in another one-fourth of that 

 quantity, yet it was sufficient to colour a large mass of 

 liquid. Doubtless many of the colours found in quartz, 

 which were very brilliant and beautiful, were due to equally 

 small proportions of foreign substance diffused through 

 it. Opal presented, in addition to the quality of opalescence, 

 which was shown in nearly all forms of opal, a wonder- 

 ful play of colours. There was every reason to believe 

 that that wonderful play of colours was produced by the 

 presence of excessively minute foreign particles, which 

 gave rise to the wonderful phenomena known as inter- 

 ference colours. 



While speaking ot the colours of these sub- 

 stances he might point out that they had there suffi- 



