360 



NATURE 



[Feb. 19, 1885 



e immon with the savages than with us, or may have been 

 specially prominent in those selected for experiment. 



Gildown, February 16 J. Rand Catron 



Erosion of Glass 



Some time in the end of 1SS2 Surgeon-Major Biden, writing 

 from Madias, related in Nature that certain glass vases on 

 which white-ant 111ml had been deposited had been eroded over 

 the area of deposit in such a way as to suggest that an acid 

 having, like hydrofluoric acid, a power of dissolving glass, was; 

 present in the "mud." On reading this I was reminded of the 

 observations of my teacher, Mr. George Rainey, recently 

 deceased. 



Mr. Rainey, in the course of his observations on molecular 

 coalescence, had shown that when carbonate of lime was de- 

 posited in spherical forms on the surface of a glass slide in the 

 presence of a strong solution of gum, the glass was eroded at 

 every point of contact of a sphere. He explained the pheno- 

 menon, as I believe rightly, by the principle of molecular coal- 

 escence. In the embrace of the colloid gum, the molecules of 

 the glass adjoining the spheres were drawn into the spheres, and 

 a little cup corresponded to each sphere-contact. There was 

 certainly no question of the action of an acid, the solutions used 

 being distinctly alkaline. 



Inspection of the bottles in which the substances have been 

 kept will show that carbonate of lime, moist or dry, carbonate 

 of potash, moist or dry, chloride of calcium, moist or dry, do 

 not in the absence of colloids erode glass. It appeared to me 

 probable that the white-ant mud must consist of a mixture of 

 some colloid with carbonate of lime or some other salt capable 

 of taking spherical form. I wrote to Surgeon-Major Biden 

 stating the possibility as it appeared to me, and suggesting that 

 the mud should be examined as regarded colloid and earthy 

 matter. He replied most courteously that the mud was not at 

 the time to be obtained, but sent some of the earth which formed 

 its basis. 



Experimenting with this earth alone, I was not able to etch 

 glass. But in view of some interesting speculations which this 

 episode started for me, I have since made some experiments 

 directly bearing on the possibility of the erosion of glass surfaces 

 by saline matters of alkaline reaction deposited on them within 

 a colloid bed or matrix. 



I inclose for your inspection a glass slide which has been so 

 treated. More than a year ago this slide was coated with a layer 

 of paraffin, melted on. The word "Ant" was drawn on the 

 side with a wood point, in the expectation that etching might be 

 effected where the paraffin was removed, the wood point being 

 incapable of scratching the glass. The expectation was not 

 entirely fulfilled. The paraffin, not being sticky enough, scaled off 

 in sheets so as to leave the whole surface ultimately exposed- 

 This whole surface is now seen to be etched. At first sight the 

 glass looks as if it were covered with a semi-opaque deposit. 

 l!ut it has been boiled in hydrochloric acid and in water, with- 

 out any change becoming evident, and under the microscope 

 the appearance rendered is clearly an appearance of erosion. 



The details of the experiment were as follows : a strong solu- 

 tion of gum arabic in distilled water was made and filtered. It 

 was divided into two portions. To one was added a small quantity 

 of chloride of calcium, to the other a small quantity of carbonate 

 of potash. A wide-mouthed bottle, three inches in height, was 

 half filled with the first solution, and the second solution was 

 slowly poured on the top of the first, so as to avoid mixture of 

 thp two. The slide, prepared, as already described, was placed 

 vertically in the bottle, so that the middle region of the slide 

 corresponded to the level of the meeting of the two solutions. 



The slide was found, at the end of a twelvemonth, denuded 

 of its paraffin, and coated with an incrustation of carbonate 

 of lime most dense at and near the meeting level of the two 

 solutions. 



Under the microscope the surface of the slide presents many 

 kinds of erosion — spherical, linear, and intermediate. But in 

 proportion as higher and higher objectives are used, all the 

 appearances are shown to be of circular form, the lines, for 

 instance, being resolved into lines of circular pits. 



I dare not make this letter too long, and therefore include in 

 it only so much as bears on Surgeon-Major Biden's most inter- 

 esting communication. It suffices, at the moment, to indicate 

 that the surface of a glass slide may be eroded in a way to 

 suggest the action of an acid, such as hydrofluoric acid, when 

 no free acid is present ; and that erosion may occur when the 



glass is brought in contact with alkaline fluid, a colloid, and 

 crystalline substances capable of assuming, in the presence of a 

 colloid, spheroidal form. 



I propose to state the results of this and other experiments, 

 and some speculations suggested thereby, before the Royal 

 Microscopical Society. William M. Ord 



7, Brook Street, W. 



Echium Crossing 



The gardens of Madeira are remarkable for the neglect of 

 native plants. This is due in part to indigenous indifference, 

 and also to a preference for familiar forms amongst people who 

 migrate hither from various regions, though chiefly to the 

 temptation to test the facilities of growth and naturalisation in a 

 moist and equable sub-tropical climate. Hence it is often 

 easier to import species peculiar to Madeira than to find them 

 in their native place ; but none the less do these rocks abound 

 with conspicuous examples of interesting genera. 



I have cultivated for many years two large echiums upon the 

 terraces of the Luinta do Valle, 300 feet above the sea, namely, 

 E. fastuosum, the Madeiran littoral species, a perennial shrub 

 3 or 4 feet high, with hairy light green leaves and branching 

 stems crowded with scorpioid racimes of light-blue flowers with 

 white stamens. And secondly, E. simplex, the giant Canarian 

 species maturing in Madeira in the second year. This remark- 

 able plant has large, smooth, silvery leaves, and terminates its 

 growth in one unbranched stem densely packed with folded 

 llower-stalks bearing pure white blossoms, and forming a 

 pyramid reaching sometimes 14 feet in height. E. simplex dies 

 after flowering. The flowers in both species last from three to 

 five weeks, and the unfolded'flower-stalks measure 2 to 3 inches 

 in length. 



Until 1882 the two echiums, though growing together and 

 having their scentless flowers freely visited by bees and insects 

 for their abundant nectar, had remained distinct ; but, in 1883, 

 after introducing a swarm of Ligurian bees from England, I 

 found that a cross-fertilisation had been effected, which has left 

 me very few examples of E. simplex. 



The hybrid Echium possesses the leaves of the giant plant, 

 and the stem merely bifurcates or branches sparingly. The 

 flowers are tinged light blue, and the perennial habit of E. 

 fastuosum is expressed by a continual growth of the flower 

 racimes, which, after flowering for two years, measure 26 inches 

 in length, and are still unfolding. The seeds of this hybrid 

 have not germinated. 



I am now preparing to effect a cross between E. simplex and 

 the handsome mountain E. cauJicans of this island at my country 

 residence, 2000 feet above the sea. 



E. caudicans and E. fastuosum have frequently blended, pro- 

 ducing plants less new in structure than in habit ; but such 

 hybrids have been quickly lost, either in sterility or reversion. 



Madeira, January 26 Michael Grabham 



[This is an interesting case of the spontaneous appearance of 

 a hybrid between two very distinct species. The occurrence of 

 such hybrids is frequent in some genera, such as Verbascum and 

 Primula, and gives systematic botanists much trouble. There 

 is a striking picture of Echium simplex at Teneriffe, in the 

 North Gallery at Kew, No. 23. — Ed.] 



The Iridescent Clouds 

 The coloured fringes and bows described by Mr. N. in Prof. 

 C. Piazzi Smyth's communication (p. 316) are clearly of a totally 

 different character from the iridescent clouds that were so widely 

 remarked in December. I take the "fringes and bows in 

 circles " mentioned by him to be simply the same phenomenon 

 of coloured circles that is so often seen around the moon, which 

 goes by the name of a " corona" ; and the reason why it is not 

 easily seen around the sun, except by reflection in glass or water, 

 is that the sun is too dazzling to look at directly. There is 

 another phenomenon of coloured clouds which is probably also 

 alluded to by Mr. N., and that is when thin clouds, usually 

 cirrus, show interference colours, often very vividly ; the 

 positions of these colours evidently depending on the structure 

 of the clouds, and being quite irregular with reference to the 

 sun. The iridescent clouds recently observed no doubt owe 

 their colour to the same cause, but the kind of cloud was evi- 

 dently different, and the colours produced were much more 

 striking. The clouds themselves were quite recognisable as 



