On Erosion of the Surface of Glass, &g. By Dr. W. M. Ord. 763 



solutions of gum containing respectively carbonate of potash and 

 chloride of calcium were superimposed one upon the other after 

 Mr. Eainey's plan. Glass slides were coated with solid paraffin ; 

 the letters of the word ANT were inscribed on them, with a piece 

 of matchwood which easily removed the paraffin without injuring 

 the surface of the glass. The slides were then placed in the gum 

 solution as in Rainey's method. In a second experiment, albumen 

 was used in a similar way. In a third, glycerin was used in a 

 similar way. In a fourth, a slide coated with paraffin and subse- 

 quently marked with the letters ANT was laid horizontally. Over 

 the exposed surface a mixture of egg-albumen and glycerin was 

 smeared to the depth of 1/10 of an inch ; then a few drops of a 

 strong solution of chloride of calcium were placed in contact with 

 one end of the islet of glycerin and albumen, and a few drops of a 

 solution of carbonate of potash in contact with the other end, so 

 that the two solutions would diffuse through the mixture of 

 glycerin and albumen, would meet therein, and produce, by 

 mutual decomposition, carbonate of lime. Glycerin was added to 

 diminish evaporation and to aid the contact of the colloid with the 

 exposed surface of the glass. Evaporation was further opposed by 

 placing the preparation in a moist chamber. Lastly, a controlling 

 experiment was made by placing a glass slide, similarly coated and 

 inscribed, in a mixture of pure glycerin and carbonate of potash. 

 In parallel experiments, slides of mother-of-pearl, and of ivory, 

 coated with paraffin and marked like the others, took the place of 

 the glass slides. In all the experiments large stoppered bottles 

 were used, so that no evaporation was possible. It was not till the 

 end of a twelvemonth that the bottles were opened and the slides 

 examined. Inspection showed that the paraffin had not been an 

 efl'ective insulating substance in the case of the glass shdes. It had 

 peeled off from them in flakes, and had floated away, so that their 

 whole surface was encrusted, in all the experiments except the last, 

 with a deposit of carbonate of lime. The carbonate of lime was 

 found to have assumed the form of small spherules closely aggre- 

 gated in dense masses and much deformed by mutual reaction. 

 Mr. Kainey in his experiments had used exceedingly dilute solutions 

 in order to obtain his beautiful spherules. To imitate the ant-mud 

 I had used much stronger solutions. But the result was that I 

 obtained a very much more complete etching of the glass, and had, 

 as I suppose, fairl)' imitated what Surgeon-Major Bidie had seen. 

 The slides which I show to-night are, 1 think you will say, deeply 

 etched. They represent the etching effected by carbonate of lime 

 in the presence of gum, of albumen, and of albumen and glycerin 

 together. I may note in passing, that pure glycerin in combina- 

 tion witli carbonate of potash produced no etching. 



Under the Microscope the etched surface of the glass shows 



