1815.] Astronomical and Magnetical Olservations, 51 
quantity for chemical analyses. The difficulty is increased when 
either much more or much less is employed. In all my analyses J 
have this rule before my eyes: “ Endeavour to find a method of 
analysis so that the accuracy depends as little as possible upon the 
manual dexterity of the operator. When this js found, consider 
what unavoidable circumstances intervene to render the results in- 
accurate, and whether by their means the quantities obtained are 
increased or diminished. “ Then make a second analysis, in which 
all these circumstances act in a way directly contrary.” If the results 
agree, the experiment is accurate.” For example, you will find in 
my analysis of red oxide of iron that in one of my experiments I 
dissolved the iron in a weighed glass capsule, evaporated the solu- 
tion, and exposed the oxide to a red heat. Here no loss was pos- 
sible. In a second experiment I dissolved the iron in aqua regia, 
precipitated it by ammonia, washed the oxide upon a weighed filter, 
and exposed it to a red heat. Here no increase was possible ; 
nothing but a diminution could take place. But both experiments 
gave the same results. Hence I concluded that in the first experi- 
ment no increase of weight had taken Place, from the impurity of 
the acid, or the corrosion of the glass ; and that in the second no 
remarkable loss had been sustained, in consequence of the inaccu- 
racy of the method followed. By these perpetual checks I have 
learnt to look for, and to avoid, sources of inaccuracy. I have 
pointed out several of these sources in my manual of chemistry 
under the article inorganic substances. Still, however, a good deal 
depends upon manual dexterity and long practice; so that it js as 
impossible to make an accurate chemist by written directions as it is 
for one artist to make another a consummate tradesman by mere 
written rules. A clever student of chemistry might perhaps learn 
from me to attend to several small particulars, which he might not 
of himself remark, and yet are of importance. 
But my letter is already too long. BERZzELIvs, 
ed 
ARTICLE X. 
Astronomical and Magnetical Observations at Hackney Pick. 
By Col. Beaufoy. 
Latitude, 51° 32! 40°3” North, Longitude West in Time 68856 
Jone 10, Occultation of » Cancri Immersion,... 9) 26’ 11/) Mean Time at 
by the moon ...., ore ee re Emersion .... 10 9 34 H, W, 
June 11, Emersion of Jupiter’s £119 $3’ 19” Mean Time at Hackney Wick, 
Ist Satelite... ....ececceese V Lh 33. 26 Ditto at Greenwich, 
D 2 
