Section of Chemistry and Electricity. 409 



Two other chemical facts were adduced. 



1. As the precipitation of the ammoniacal phosphate of mag- 

 nesia is accelerated and made manifest by drawing lines with a blunt 

 glass rod on the internal sides of the glass vessel in which the proper 

 mixture is made for producing the precipitate, a fact first noticed 

 by the late Dr. Wollaston, so in a similar manner the separation 

 of bitartrate of potash from any mixture containing potash, to which 

 tartaric acid has been added in proper quantity, will be accelerated 

 and rendered manifest by drawing lines with pressure on the inter- 

 nal sides of the vessel with a glass rod, the crystals of bitartrate first 

 attaching themselves to these lines. 



2. That nitrate of lead like the nitrate of baryta is precipitated 

 from water by addition of strong nitric acid, which in each case ex- 

 erts a similar action, namely, that of abstracting the water from the 

 salt. 



On a Source of Inaccuracy in Observations of the Dew-point. By the 

 Rev. Wm. Vernon Harcourt, F.R.S. 



Mr. Harcourt having observed an apparent variableness in the 

 deposition of dew on different surfaces, at the same temperature 

 and in the same atmosphere, was led to make the following experi- 

 ments. 



A pane of glass was rubbed, on different portions of its surface, 

 with substances of different degrees of hardness, and left till the 

 equality of temperature was restored : being then breathed upon, it 

 was observed to show the condensed vapour in proportion to the 

 polishing power of the substances by which the different parts of the 

 glass had been rubbed ; characters traced by a leaden point dis- 

 played this phenomenon in the greatest perfection. The experi- 

 ment was next tried on metallic surfaces, by polishing, for instance, 

 part of the blade of a rough razor, and breathing on it, when the 

 same effect was obvious. 



When the state of the dew on the different surfaces was examined 

 with a lens, it appeared that its greater visibility on the more po- 

 lished parts was owing to a stronger reflection of light from a greater 

 number of minute and unconnected drops deposited on those parts. 

 It would seem as if the process of polishing insulates the points to 

 which the particles of vapour attach themselves, and prevents them 

 from running into each other ; but though the vapour condensed on 

 the polished surface thus becomes more sensible, it is not increased 

 in quantity, as is easily proved by continuing to breathe on the pane 

 of a window till streams of water run down on the unrubbed surface, 

 and on that only ; and it is not a little remarkable, if the polish be 

 also carried horizontally along the lower part of the pane, to observe 

 the streams dammed up where they meet the polished part, and 

 drops of water left along that portion of the line. 



The observation of these facts led the author to apply some prac- 

 tical corrections to the ordinary method of ascertaining the dew- 

 point : he adopts the direct process of Dr. Dalton, reducing the 



Third Serie*. Vol. 7. No. 41. Nov. 1835. 3 G 



