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XV. — On Erosion of the Surface of Glass, when exposed to the 

 Joint Action of Carhonate of Lime and Colloids. 



By William M. Ord, M.D., F.E.M.S., F.L.S. 



(Eead Uth March, 1885.) 



More than a year ago there appeared in ' Nature ' a letter from 

 Sui'geon- Major Bidie, now Sanitary Commissioner, Madras, 

 describing a sort of etching of some glass vessels at points on which 

 white-ant mud had been deposited. 



There was much in this letter to arouse my interest. The 

 story made it probable that the deposit of ant-mud had somehow 

 eaten away the polish of the glass. The first explanation of the 

 erosion was found, plausibly, in the hypothesis of the presence in 

 the ant-mud of an acid capable of dissolving glass after the fashion 

 of hydrofluoric acid. But no organic acids are known to possess 

 such a property, and the presumable presence of some colloidal 

 organic matter in the ant-mud led me to seek an explanation of a 

 very different kmd. 



The observations and reasonings of my distinguished teacher, 

 the late Mr. George Eainey, had been always fresh in my mind. I 

 knew that in some of his experiments he had, incidentally, noted 

 an erosion of glass surfaces wherein no free acid or alkali was 

 concerned. At the risk of wearying the members of this Society 

 by the repetition of a story already well known to them, I will 

 briefly recapitulate the observations to which I refer. Mr. Eainey 

 was engaged in investigating the causes leading to the assumption 

 by carbonate of lime of a spherical form in various solutions. In 

 some of his experiments he allowed two solutions of gum, one 

 containing carbonate of potash, the other containing carbonate of 

 calcium, to mix slowly ; the one being superimposed upon the 

 other. Next he introduced glass slides of the kind ordinarily used 

 for microscopic purposes, into the apposed solutions, with the 

 expectation, confirmed by the result, that a deposit of carbonate 

 of lime would be presently formed on theu' surfaces. After pro- 

 longed immersion under conditions which precluded the occurrence 

 of evaporation, the glass slides were found to have become 

 covered with adherent spherules of carbonate of lime. The adhesion 

 was of considerable firmness. The spheres remained attached after 

 washing with water ; and where their complete removal was sought, 

 the use of diluted hydrochloric acid was necessary. It was then 

 observed that the surface of the glass was no longer smooth and 

 transparent. Mr. Eainey took casts of the surface with collodion, 

 and was able to show from examination of the casts that the loss of 

 traasparcDcy was brought about by the formation of shallow 



