1829.] On the Manufacture of Optical Glass. 265 



rimental room in which a powerful furnace was at work, were 

 competent to do much harm in eighteen or twenty-four hours, 

 by giving colour and forming striae, experiments were made on 

 the means of cleansing the entering air. It was found easy to 

 effect this, by the assistance of two or three Woulfe's bottles, 

 or two or three jars, inverted one within another, using at the 

 same time portions of diluted sulphuric acid, or such solutions 

 of salts in the vessels as would not supply any moisture to the 

 air, but rather take water with the dust from it. In these cases 

 the air did not bubble through the liquid, but only passed close 

 to its surface, and had time to deposit its dust during its pass- 

 age through the enclosed spaces above the fluid ; but finally 

 a still simpler arrangement was used, consisting merely of a 

 plug of clean dry sponge fitted into the end of the tube, which, 

 at the same time that it allowed sufficient air to pass, seemed, 

 from the appearance of the tube afterwards, to have excluded 

 every impurity. 



71. There are two conditions of the finished glass, each of 

 great importance, which it is the object of the process to secure 

 in this state of the substance. One, and the most essential, is 

 the absence of all striae and irregularities of composition ; the 

 other, the absence of even the most minute bubbles. The first 

 is obtained by agitation and perfect mixture of the whole ; the 

 latter, principally by a state of repose : so that the means re- 

 quired to be successful on both points are directly opposed to 

 each other. Were the glass absolutely incapable of change by 

 the long-continued action of heat, it would be easy first to ren- 

 der it uniform by stirring, and then to leave it in a quiescent 

 state, until the bubbles had disappeared ; but I am not yet fully 

 assured of the fact which is necessary to this order of proceed- 

 ings. That the glass, as far as proportions are concerned, if 

 changed at all, is altered only in an extremely minute and inap- 

 preciable degree, is shown by some experiments, in which, after 

 a portion had been prepared and heated for many hours, and 

 also stirred well, the resulting piece was divided into smaller 

 portions, and these heated at different temperatures, in platinum 

 trays, for sixteen hours. Three portions were heated as power- 

 fully as the furnace would admit of; three only to redness, which 

 may be considered as a very low heat ; and three to an inter- 

 mediate degree : all were cooled slowly and annealed for an 



