60 



The cotton wool, if necessary, can be kept moist by taking another 

 flask, filling it with water, dipping one end of a lamp cotton into 

 the water, and aUoAving the other end to rest on top of the plug 

 of cotton wool. Fluoride of Silicium, or, as it is now called silicic 

 fluoride, will slowly rise in fumes from the mixture of fluor spar, 

 glass, and sulphuric acid, and deposit a portion of the silica in the 

 form of vesicles, very much resembling in shape sausage skins. 

 Some of these vesicles, on examination, Avill be foimd to be quite 

 plain, while others -^vill be elegantly marked. 



Before attempting to examine the vesicles, it is necessary to 

 well wash and dry them, otherwise the moisture from them will 

 corrode the glass of the objective, or damage the brass-work of 

 the microscope. 



They may be examined as opaque objects under a low power 

 to obtain an idea of the general form and appearance of the vesicles. 

 They may then be gently crushed on the slide under a thin 

 covering glass, and examined with a high power ranging from a 

 quarter upwards. Some will be foimd to bear a striking re- 

 semblance to the valves of diatoms in their marking ; while others 

 Anil exhibit large (comparatively) round bosses, with smaller bosses 

 grouped around them. Max Schultze in his article suggested that 

 the hemispherical and other markings were due to crystallization. 

 Tliis may possibly be the case. We certainly have examples of 

 silex depositing itself in form of bosses and knobs, as Avell as tliose 

 peculiar concretions mentioned by Mr. Paukhurst in liis paper on 

 " Tlie Forms of Silica." We have also noticed similar minute 

 bosses in pieces of disintegrated Eoman glass from the Catacombs. 



An interesting discussion followed, in which most of the 

 Members present joined, after Avhich objects illustrative of the 

 subject of the paper, includiDg real diatoms, chalcedony,- disinteg- 

 lated glass from the Catacombs, and crystals in glass Avere sheAvn 

 by the President (Mr. G. D. Sawyer) and Messrs. Ajlen, Glaisyer, 

 Haselwood, and Wonfor. 



