CHEMISTRY. 
matter, either by heating it, or by freezing 
tlie surplus water and taking out the ice ; 
10^ when evaporation is performed in any 
apparatus of vessels, partly or quite closed, 
and the vapours, after being raised by heat 
in one part or vessel, are received in an- 
other sufficiently cold to condense them 
into the fluid slate, this process is called dis- 
tillation ; II, when a fluid obtained by 
distillation is again distilled, in order to 
obtain the most volatile part of the first 
product, this last part is said to be rectified, 
and the process is called rectification. This 
term has become nearly obsolete in scien- 
tific description, but is still retained in the 
Arts; 12, there are many products of eva- 
poration which congeal, or become solid 
at a temperature much higher than that of 
the atmosphere, and are not, therefore, ob- 
tained in the fluid, but the solid state. 
These usually adhere in the form of crystals 
to the upper part of the apparatus ; and on 
this account, as well as because the opera- 
tion does not in general require the same 
kind of vessel, it is distinguished by the 
name of sublimation, and the products 
themselves are called sublimates, and in 
some instances flowers ; but these two last 
terms are more particularly confined to 
the Arts. Other terms are also used, such 
as fusible, evaporable, &c. but their sense 
is manifest. 
For the apparatus used in these and the 
otlier operations of chemistry, see Labo- 
ratory. 
The consideration of what happens to 
the parts of bodies, in consequence of their 
elective, attractions, constitutes the most 
difticult part of the science, whether the 
mind be employed in developing the facts, 
or in deducing the general theory which 
may be indicated from them. It is, there- 
fore, necessary to consider them with some 
attention, and in a regular manner. 
The adliesion of parts, considered to be 
of the same kind, is called aggregation. 
Thus a number of pieces of glass melted 
together form an aggregate 5 and the 
smallest parts into which an aggregate can 
be imagined to be divided, so as not to 
change its nature, are called integrant 
parts ; so that the integrant parts of glass 
are themselves glass. But when tlie body 
is known to be made up of parts of diffe- 
rent kinds or nature, and it is considered 
with regard to these, the body is called a 
compound, or combination, and the parts 
are called component parts, or principles. 
In this manner glass is a compouud of the 
earth called silex, and a salt called alkali*, 
combined together at a sti ong heat : and 
we may imagine that if tliCfre Were any 
means by Which glass could be ffediiced, 
first to its integrant parts, and the division 
could be carried farther, the parts would 
then be no longer integrant and glass, but 
would become divided into component 
parts, namely, earth and alkali. Bodies 
are also considered in a wide manner by 
the name of mixtures, when small aggre- 
gates of different kinds are united, as in 
a variety of minerals where the parts arC 
frequently distinguishable by the senses : 
and in the arts we have sand and lime 
made into mortar by mixture, or sand, 
clay, and other earths, made into pottery, 
and hardened by a moderate fire ; but 
these by a stronger heat may be made to 
combine into glass, and are then no longer 
mixtures, but compounds. 
The early chemists were led into a sup- 
position, that the bodies they were unable 
to analyze were simple, and they distiil- 
guished them by the name of elements. 
It is probable that the great variety of 
bodies around us are formed, by combina- 
tion, out of a few simple principles, or per- 
haps out of one single element, variously 
combined as to figure and position of 
parts ; but it is useless and unprofitable to 
speculate on probabilities, which experi- 
ment can never verify. Modern chemists, 
very properly, consider those bodies as 
simple which have not yet been decom- 
posed ; but this is merely with relation to 
the present state of our knowledge, and 
for the sake of arrangement and induction. 
They do not lose sight of the necessity of 
instituting experiments for their farther 
analysis ; and the great discoveries which 
have done honour to our own times are a 
proof of their diligence and sagacity. 
We do not know of any means of ascer- 
taining by experiment,- whether compound 
bodies do enter as principles into other 
bodies still more compounded; or whether 
in bodies of three or more principles, all 
the simple particles do dispose themselves 
without any dependahee on the order of 
time, according to which they may have 
been put togetlier. It is probable that 
the former is the case, so tlrat we may 
hereafter be enabled to designate primaiy 
principles, or bodies not yet decomposed ; 
secondary principles, or bodies of two pri- 
mary principles, which nevertheless can 
enter into combination, or be disengaged 
without separation from each other; ter- 
