42 Notes on Barometer Construction. 



Dictionary of Watts, article "Barometer," will be found an 

 account of the formation of the large bore barometer of the 

 Kew Observatory ; it will there be seen that the tube was 

 polished out with alcohol and whiting (precipitated chalk, 

 probably). Fuming nitric acid is an efficient oxident of 

 greasy substances, and immersion of tubes in this acid before 

 the final polishing, or first in oil of vitriol and next in nitric 

 acid, would conduce to a satisfactory result ; but whatever 

 be done in the way of polishing out the tube, extreme care 

 in avoiding the slightest scratch or abrasion of the inner glass 

 surface must be observed. If iron wire be used for carrying 

 the polishing plug, the wire must be covered completely with 

 lamp cotton ; the latter should have been previously purified 

 by digestion in ether or bisulphide of carbon. But even with 

 these precautions there is a risk of filaments, and perhaps, on 

 the whole, it is best to avoid covered wire altogether. Brass 

 or copper wire are less dangerous, but whalebone, or cane, or 

 soft non-resinous or de-resinated woods have some peculiar 

 recommendations. 



I here may point out in reference to the cleaning of glass 

 tubes generally, and especially to the cleaning of curved 

 tubes with complex bends, and when whalebone of sufficient 

 length is obtainable, that it possesses a property which can- 

 not be too pointedly indicated to those who have not hitherto 

 recognised it, and who are engaged in experimental physics. 

 By its means some problems in cleaning the interior of com- 

 plex forms of glass vessels can be solved which, to the best 

 of my knowledge, are soluble by no other known means. A 

 rod of whalebone is taken and shaped to our requirement ; 

 we intend to pass it through certain tubular crooked ways to 

 reach a certain point on some remote inner surface; the material 

 is elastic enough to pass through the tortuous duct, but when 

 this is accomplished we have little or no control over the 

 inner end of the slight constrained whalebone rod on which 

 we depend for doing the work. But the possibility of doing 



requirement, it is still important to point out the difference between modes 

 quite effective for lecture table demonstration, and those to be observed 

 in the construction of instruments intended to meet all the require- 

 ments of precise physical research. Indiarubber finger stalls, collodion 

 films, gutta-percha moulded valves, and similar contrivances, suggest them- 

 selves ; but without attaching weight or preference to any of these, it still 

 remains as a fact worthy of our best attention, that we cannot bring the 

 hand into contact with pure mercury or chemically pure glass without in 

 some measure fouling their surfaces^ 



