436 On the fundamental Property of the Lever. 
barytes a c@émpound soluble in water; it underzacs some 
alteration while the solution is evaporating ; it appears that 
the sulphuric acid combined with it, re-acts upon the vege- 
table matter and blackens it. The sulphuric acid cannot 
be separated from the astringent matter without decom- 
posing it. 
In the analysis of vegetables, we are not to conclude that 
whenever any substance precipitates gelatin, that substance 
is tannin; for it is very probable, that there are several 
substances very different from each other, which possess 
that property. Lastly, since we find, Ist, that for the most 
part, those substances which form a precipitate with ge- 
latin are acid; 2d, that frequently this animal matter can- 
not be precipitated from vegetable infusions, without -the 
addition of an acid; and, 3d, that the greatest number ot 
natura] tannins redden turnsole;. we may fairly presume 
that tannins are combinations of vegetable acids with sub- 
stances of various natures. 
7 
LXXII.° Demonstration of the fundamental Property of the 
Lever. By Davip Brewster, LL.D. F.R.S. Edin.* 
Iris a singular fact in the history of science, that, after 
all the attempts of the most eminent modern mathemati- 
cians, to obtain a simple and satisfactory demonstration. of 
the fundamental property of the lever, the solution of this 
problem given by Archimedes should still be considered as 
the most legitimate and elementary. Galileo, Huygens, De Ja 
Hire, Sir Isaac Newton, Maclaurin, Landen, and Hamilton, 
have directed their attention to this important part of me- 
chanics; but their demonstrations are in general either 
tedious and abstruse, or founded on assumptions too arbi- 
trary to be recognised asa proper basis for mathematical 
reasoning. Even the demonstration given by Archimedes 
is not free from objections, and is: applicable only to the 
lever, considered as a physical body. Galileo, though his 
demonstration is superior in point of simplicity to that of 
Archimedes, resorts to the inelegant contrivance, of sus- 
pending a solid prism from a mathematical lever, and of 
dividing the prism into two unequal parts, which act as the 
power and the weight... The demonstration given by Huy- 
gens assumes as an axiom, that a given weight removed 
from the fulerum has a greater tendency to turn the lever 
. * From the Transactions of the Royal Society of Edinburgh. ; 
round 
