164 
combinations, the nature and composition of which 
are well known. 
The reverse process of analysis is synthesis, by 
which we again produce a substance from its 
component parts. But as certain substances often 
combine only under peculiar circumstances, it 
will be evident that, unless we know these cir- 
cumstances, or are able to contrive them, the 
synthesis of the substance under such circum- 
stances will remain impracticable, but does not, 
therefore, in the slightest degree, impair the cor- 
rectness of our analysis. 
Chemical analysis requires, more than any other 
branch of applied chemistry, a thorough know- 
ledge of the science of chemistry; but it is, on 
the other hand, itself the base of all chemical 
knowledge, since it alone enables us to investi- 
gate the changes which take place whenever 
chemical affinity is called into action, and to as- 
certain the laws which govern them, by ascer- 
| taining the exact composition of the resulting 
compounds. The progress of the science of chem- 
istry, and the perfection of analytical chemistry, 
| have, therefore, always kept pace with each 
other. The following are the general principles 
of chemical analysis. A substance is either a 
simple body, an element, or a chemical combina- 
tion, or a mechanical mixture of them. An ex- 
ample will illustrate this. Hydrogen and chlo- 
rine gases are both elementary bodies, viz. such 
as chemistry is not capable of further separating 
into others, but which may be recovered again 
from all combinations in which they exist. If 
| these be mixed in equal volumes, we have a me- 
chanical mixture of two elementary bodies, which 
again may be separated without the aid of chem- 
ical means, as by mere absorbents. But if the 
mixture be exposed to the sun’s rays, they sud- 
denly enter into chemical combination, which is 
made apparent by a flash of fire passing through 
the whole mass. The gas suffers hereby no change 
in volume nor in weight, but, on examination, it 
will be found to have entirely different properties 
from either hydrogen or chlorine, or their mix- 
ture; 1t is an entirely different gas, called chlo- 
rohydric acid gas, from which neither of its con- 
stituent elements can be separated by any merely 
mechanical means. If any of the two gases were 
| added in a larger proportion than equal volumes, 
we should still have the same occurrence, but in- 
stead of the whole of the gas being converted 
into chlorohydric acid gas, we should have this 
latter gas, mechanically mixed with the gas added 
in excess, or a mechanical mixture of a chemical 
compound with an element. 
A mechanical mixture may often be separated 
by mere mechanical means into its component 
parts, which may then be recognised as sub- 
stances of known chemical composition. Thus, 
| a mixture of a light and a heavy powder may 
often be separated by washing with water, which 
will float off the lighter. Although, in such cases, 
where we employ purely mechanical means to 
ANALYSIS. 
separate mixtures into their component parts, 
the operation ought strictly not to be called 
analysis, at least not chemical analysis; still or- 
dinary language does not draw this distinction 
principally where the quantitative estimation of 
any of the component parts is the main object. 
We might thus speak of the analysis of a mag- 
netic iron ore, when it is pulverized, and the ore 
separated from the gangue by extraction with a 
magnet, in order to determine their relative quan- 
tity. The modern improvements in the micro- 
scope, and its use, afford to chemistry an invalu- 
able means of discovering component parts, where 
they exist merely as mechanical admixtures, when 
chemistry alone would be unable to decide it. 
Thus, the long-contested question among chem- 
ists, whether kermes mineral contained oxide of 
antimony, as.an essential or chemically combined 
part, was decided by the aid of the microscope, 
by showing the oxide of antimony to exist in it 
in the state of exceedingly minute crystals, and 
therefore not in chemical combination with the 
sulphuret of antimony. Mechanical mixtures 
may further be separated by mere absorbents or 
perfectly neutral solvents, which take up some 
of its parts and leave others behind; and if, on 
the other hand, a substance be taken up from a 
mixture by a solvent, to which it otherwise is | 
perfectly indifferent, or has no chemical affinity, | 
there is good reason to believe that it existed 
merely as mechanical admixture. Or some or 
all the ingredients may be taken up by a solvent | 
and afterwards separated by crystallization, or 
by precipitation, by the addition of other per- | 
fectly neutral solvents to the first. 
Mechanical mixtures may also be analyzed by | 
more purely chemical means, such as dissolving | 
reagents, if some of the admixtures be acted on | 
by them, while others are not affected. If we | 
thus treat a mixture of carbonate of lime and 
silica with a dilute acid, the latter dissolves the 
lime, with expulsion of the carbonic acid, while 
insoluble silica is left behind. A very consider- 
able number of what are commonly called chenvw- 
cal analyses are nothing more than the separation | 
of mechanical mixtures by the two last-mentioned 
means: thus, a great many analyses of parts of 
plants or animals, by which they are separated 
by different solvents into different neutral sub- 
stances, such as sugar, starch, gum, lignin, vola- 
tile oil, &c., are nothing more than a separation | 
of a mechanical mixture of these substances. 
Heat and other such agents may.also be often 
used for the same purpose: thus, mixtures of 
different fatty matters of different fusibility, 
may be separated by cautious fusion; or liquors 
of different volatility by distillation. It is most- 
ly in analysis for scientific purposes that it 
is of importance to ascertain whether a sub- 
stance is a pure chemical combination or a 
mechanical mixture of several compounds. For 
most practical purposes the main object is merely. 
to find the ingredients, it being of little or ‘no 
| 
