62 Canadian Record of Science. 



action of calcium, carbonate and colloids. The investigation was 

 suggested by a note contributed by Dr. Bidie, of Madras, to Nature, 

 xxvi. 549. (1882), describing an etching of glass vessels where 

 white ant mud had been deposited. The particular direction of 

 the experiments was largely suggested by the previous investiga- 

 tions of Mr. Rainey, who found that, when two separate solutions 

 of gum — the one containing calcium carbonate, the other contain- 

 ing potassium carbonate — were allowed to mix slowly, and a 

 glass slide was then introduced, a deposit of carbonate of lime was 

 soon formed upon the surface of the slide, the incrustation being 

 composed of minute spherules which adhered with considerable 

 firmness. After removal of these spheres by the action of dilute 

 hydrochloric acid, the surface of the glass was found to be some- 

 what opaque. From collodion casts, Mr. Rainey further proved 

 that the opacity was due to minute depressions in the glass cor- 

 responding to the position of the carbonate of lime spherules. The 

 formation of these bodies was shown to originate in minute gran- 

 ules, each with its own centre of attraction, but as they became 

 more aggregated, the centre of attraction for each granule becam e 

 replaced by one centre of attraction common to all within a certain 

 radius, thus giving one spherule by agglomeration of numerous 

 granules. Extending this hypothesis to the etching of the glass, 

 Mr. Rainey argues that when such a spherule was found on a glass 

 surface with the surrounding colloid gum sticking to the glass, and 

 actually entering also into the composition of the sphere, the same 

 attractive power which had determined the incorporation into one 

 sphere of a number of spherules in contact with one another, would 

 determine also the incorporation of adjoining molecules of glass 

 into the incumbent sphere. Thus a pit would be found opposite 

 each sphere. 



Following up those conclusions, Dr. Ord coated a series of slides 

 with paraffin, albumen, and glycerine, one each. These bore cer- 

 tain inscriptions cut through the coating so as to expose the sur- 

 face of the glass. Slides of mother of pearl and of ivory were sim- 

 . ilarly treated, and all were then brought in contact with calcium 

 chloride and potassium carbonate in such a way that evaporation 

 was arrested and the two salts would gradually mingle over the 

 coated surface of the slide. The results obtained at the end of 

 twelve months showed the formation of spherules of calcium car- 

 bonate, in some cases densely aggregated, and a corresponding 

 etching of the surfaces exposed to their contact. 



The inferences drawn from these experiments are: (1) that with- 

 out the use of acids or alkalies, which are known to be capable of 

 dissolving glass, a glass surface may be eroded almost to opacity 

 when placed in contact with carbonate of lime and a colloid ; (2) 



