ANALYSIS. 
be, their presence or absence may be ascertained 
by special tests for this purpose. The reagents 
which we thus most. commonly make use of, are 
sulphuretted hydrogen and sulphohydrate of am- 
monium, to separate the metals proper; ammo- 
nia for the earths; the alkaline carbonates for 
the earthy alkalines, and the last class is gener- 
ally formed by the alkalies, The electro-negative 
elements and acids are mostly detected by tests 
applied for their special detection. 
In this way, with some practice, a tolerable 
certainty may be obtained, that no ingredient 
has escaped our notice, which is still more con- 
firmed by the subsequent quantitative analysis, 
when the joint weight of them all is found to 
equal that of the whole substance. But before 
proceeding to the quantitative estimation, it is 
generally necessary to confirm and verify the 
conclusions which we have drawn from the regu- 
lar systematic course of our examination, as to 
the presence or absence of the different ingre- 
dients, either by the application of other charac- 
| teristic tests for those we have discovered, or by 
varying our method of examination, by which 
| we might have been led into doubts or errors 
about others. 
Such confirmatory experiments 
or tests ought never to be omitted; and in the 
manuals of analytical chemistry, a large propor- 
tion of their contents is generally devoted to the 
| behaviour of the different substances with most 
of the usually employed reagents, the greater 
portion of which will be found in the present 
work, under the heads of the different substances. 
The elementary bodies themselves, when they 
occur as objects for analysis in their uncombined 
state, are recognised by their physical properties 
and their behaviour to the different reagents, by 
which they are made to dissolve and enter into 
combinations, and may then be recognised as 
other ingredients. 
After having determined what the ingredients 
are, of which a substance consists, it next re- 
mains to estimate their quantity. For this pur- 
pose one or more portions of the substance are 
employed, and the weight of the different ingre- 
dients obtained from them ascertained, and gen- 
erally expressed in 100 parts or per cent. of the 
substance. The object of quantitative analysis 
is, therefore, to liberate the ingredients in such 
a state, or to transfer them to such combinations, 
that a complete separation of them or their new 
combinations may be effected, and the estimation 
of their quantity become possible. 
The quantitative analysis—or, at least, the 
quantitative estimation of certain ingredients— 
is sometimes performed in the dry way, and is 
then termed the dry assay, or, simply, assaying. 
Thus, for many metallurgic operations, the ores 
are fused in a crucible, with certain reducing re- 
agents and fluxes, by the draft of a furnace or in 
the blast of a forge, and the quantity of reduced 
metal obtained from the ore ascertained. But, 
although these operations are not without prac- 
167 
tical value, still they are at present performed 
equally well, and with much more accuracy, in 
the moist way. The estimation of the quantity 
of gold and silver in alloys, by eupellation, is an- 
other much-practised application of the dry as- 
say; but even here it may be substituted, and 
with still greater accuracy, by the moist way ; 
and this latter is always resorted to whenever it 
becomes desirable to estimate the quantity of all 
the different ingredients which a substance con- 
tains. 
It has been stated before, that chemical com- 
binations have always the same composition. It 
is, therefore, not necessary that the ingredients 
should be weighed by themselves. Chemistry 
teaches us that when substances combine chemi- 
cally, it is in certain fixed proportions, and that, 
therefore, pure chemical compounds always con- 
tain the same amount of their different consti- 
tuents. When we, therefore, have separated an 
ingredient by transferring it to another chemical 
combination, it is only necessary to ascertain the 
quantity of this compound, and then, from its 
known composition, calculate the quantity of the 
ingredient, whose weight of the original substance 
we desired to know: thus, if a substance contain 
sulphuric acid, it would be impossible to separate 
the acid completely by itself, or weigh it, in this 
state; but if any other solution, containing bary- 
ta, be added, all the sulphuric acid will combine 
with so much of the baryta as is necessary to 
form an insoluble compound, the sulphate of ba- 
ryta, which is easily separated and weighed, and 
from the weight of which the weight of the sul- 
phuric acid is calculated. It is also evident, that 
instead of ascertaining directly the weight of an 
ingredient or its new compound, the loss in weight 
which they cause in the remainder, by their sep- 
aration or escape, may be ascertained. ‘This is 
the case, where the separation is easily effected ; | 
but the state in which it separates either renders | 
it unfit for-weighing, or its collection or prepar- 
ation for this purpose is rendered more difficult 
than the weighing of the remaining substances: 
thus, oxide of lead combines chemically with 
water: by heating this compound the water is 
expelled as vapour. It is hence easier to weigh 
the oxide of lead subsequently to the expulsion 
of the water, than to condense the vapours com- 
pletely, for the purpose of weighing them direct- 
ly; but if the oxide of lead be, at the same time, 
combined with other volatile ingredients, as, for 
instance, in common white lead, which is a com- 
bination of oxide of lead with carbonic acid and 
water, it then becomes necessary to condense and 
collect the volatilized water, and ascertain its 
weight directly. Having then ascertained the 
loss of all the volatile matter and the weight of 
the condensed water, by subtracting the latter 
from the former, we obtain the loss or weight of 
the carbonic acid. 
It often happens that two or more ingredients 
are easily separated together from the rest, but 
