54 
BRITISH ASSOCIATION.— 1835. 
has passed through, yielding its proper precipitate with the triple 
prussiate. It is right to observe, that a solution of the muriate of 
manganese is not rendered turbid by admixture with acetate of pot¬ 
ash and subsequent application of heat. 
It follows from the preceding experiments, which have been many 
times repeated, that peroxide of iron may be completely separated 
from either sulphuric or muriatic acid by acetate of potash, and that 
in a mixed solution of peroxide of iron, and oxide of manganese in 
an acid, a complete separation of the peroxide of iron may be ef¬ 
fected by means of the acetate, provided that proper attention has 
been given to the comparative quantities of the ingredients employed 
in the mixture. Acetate of soda or of ammonia mav be substituted 
%/ 
for acetate of potash in producing this decomposition. 
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 Harcqurt, 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 experiments. 
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 w r as owning to a stronger reflection of light from a greater 
number of minute and unconnected drops deposited on those parts. 
