166 
an insoluble powder; or if sulphuric acid be added 
to a solution containing baryta, the affinity of 
the latter for sulphuric acid is greater than for 
any other acid. It will therefore combine with 
it, and separate as an insoluble and easily recog- 
nisable compound, sulphate of baryta. What 
single affinity is unable to effect, may be accom- 
plished by double affinity. Thus, if oxalate of 
lime were given us for analysis, we should not 
be able to separate the oxalic acid from the lime 
by any acid or base alone; but if we treat it with 
carbonate of potash, the affinity of the potash 
for the oxalic acid, and that of the carbonic acid 
for the lime, act at the same time, and we de- 
compose it into oxalate of potash and carbonate 
of lime, in which compounds the oxalic acid and 
the lime are easily detected. 
When thus liberated, or transferred to other 
combinations, we employ various means to re- 
cognise the ingredients, or the new compounds 
to which they are transferred, as their physical 
properties, such as peculiar or striking colours, 
thus, the presence of manganese in a substance, 
is recognised by the green colour which it yields, 
' when fused with nitre and carbonate of soda, 
owing to a combination, which it always forms 
under these circumstances. Another means of 
recognition is by their peculiar odour or taste; 
thus, acetic acid, when liberated, is, in most cases, 
recognised by its peculiar odour, and the different 
kinds of sugar by their taste. The senses of 
smelling and tasting are of no less importance to 
the analytical chemist than the sight, and are 
capable of considerable cultivation. It is parti- 
cularly in organic analysis that they are invalu- 
able, as affording the only means of discovering 
many of the organic proximate constituents. Of 
no less importance are the different forms of ag- 
gregation, which the ingredients or their new 
combinations assume: as, for instance, whether 
they appear as a gas or a liquid, or an insoluble 
solid. This latter is of particular importance, 
and we therefore generally try to effect a solu- 
tion of the substance, and then add different 
other substances, mostly in solution, with which 
the different ingredients form compounds, which 
separate, and are distinguished by their different 
degrees of insolubility, or peculiar form of aggre- 
gation, such as whether they be heavy or light, 
pulverulent, flocculent, crystalline, &é. 
The substances we add for the sake of separ- 
ating or combining with the ingredients of the 
substance under examination, are called reagents. 
If they produce any change with them, they are 
said to react with them; if they form insoluble 
compounds, which separate, they precipitate them ; 
if the produced change or compound be such as 
to lead to the recognition of the ingredient, it is 
said to be a ¢est for it. Thus, when a solution of 
iodine is added to a solution containing starch, 
it produces a beautiful blue colour. As iodine 
produces this colour with no other substance than 
ANALYSIS. 
Or if we add a solution of a salt of baryta to a 
solution containing sulphuric acid, the latter pre- 
cipitates in combination with baryta, as sulphate 
of baryta, which remains insoluble by the addi- 
tion of chlorohydric acid. As no other substance 
forms a similar precipitate with barytic salts, in- 
soluble in water and an excess of chlorohydric 
acid, baryta is said to be a test for sulphuric acid. 
The insolubility of the compounds formed by the 
addition of reagents with the different ingredi- 
ents, becomes of still more importance, since it 
affords the means of removing them, either for 
the sake of further examining them, or of pre- 
venting them from interfering, by their presence, 
with the discovery and recognition of the other 
ingredients. 
In this way, all the different ingredients of a 
substance, may be recognised by adding the dif- 
ferent reagents and tests to different portions of 
the substance or by applying them successively 
to the same portion, removing, if necessary, the 
ingredients, as they are recognised. But, by such 
indiscriminate or random application of reagents 
it would not be possible to prevent some ingre’ 
dients from escaping notice. It therefore be- 
comes necessary to introduce a systematic me- 
thod of proceeding in the application of the re- 
agents. As the reagents, for their action, always 
require more or less fluidity, this may either be 
attained by heat or fusion, or by solution. Hence, 
the distinction between analysis in the dry, and 
in the moist or humid way. The qualitative ex- 
amination by the former method is generally per- 
formed more or less in connection with the blow- 
pipe, by which we fuse small beads of different 
substances, and then observe the reactions which 
take place by adding small portions of the sub- 
stance under examination, and its behaviour by 
different treatments, and with different reagents. 
For some metallurgic purposes, fusion on larger 
scale in crucibles, by the aid of furnaces, is re- 
sorted to; but although testing, in the dry way, 
and by the blowpipe in particular, has been car- 
ried to a high degree of perfection, and is per- 
formed with a great deal of facility and conve- 
nience, it cannot compare, in point of complete- 
ness or systematic procedure, with that in the 
moist way. , 
By testing in the moist way, it always becomes 
an object to effect a solution of the substance, 
and then apply such reagents as will form in- 
soluble compounds with a certain class or group 
of ingredients; then separate these, and apply 
another reagent, which will separate another. ||° 
class or group of ingredients, left in solution by 
the previous reagent, and so on: thus, all known. 
ingredients are separated into groups, containing 
only a certain number of them, which are them- 
selves distinguished or separated from each other 
by other reagents and tests. The last class or 
group is formed by those ingredients which are 
not precipitated by any of the previously applied 
starch, we say that iodine is a test for starch. | reagents, and it being known which such may 
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