VEGETABLE PHYSIOLOGY. 1S7 ^P 
bi'ief sketcli of the nature of cliemieal phenomena. The business of the chemist is to examine into 
the peculiar characters of the substances of -which the material world is composed ; to separate those 
which he finds to be made up of different constituents, so as to arrive at the elements, the first 
unalterable materials of which all things are made up ; and to seek out and define the laws which 
regulate the combinations of these elements. In common language the word element has not yet 
lost a false signification which has remained attached to it since the days when chemistry did not 
exist. The elements are said to be four, fire, aii', earth, and water; and the ancients thought the 
world was really made of these. All things may, indeed, be still said to exist only in thi-ee of these 
elemental forms, namely, of solids, liquids, and airs or gases, for irre is a result of combination; but 
this is true only in respect to form, and we are now acquainted with about sixty different substances, 
which are at present to be considered as elements, since we are unable to decompose them any 
further. Some of them are solid, others liquid, others gaseous, at the usual temperature of the 
atmosphere. Among the solids, for instance, are all the metals (except quicksilver), carbon (char- 
coal), sulphirr, iodine, &c. ; among the liquids, quicksilver and one of the constituents of sea-water, 
called bromine ; among the gases oxygen and nitrogen, which, in a state of mixtui'e, form the aii- we 
breathe ; hydrogen, which, when combined with a certain proportion of carbon, forms the common 
gas used for illumination, &e. Many of these change their form under peculiar circumstances, 
especially under different degrees of heat; thus, extreme cold will fi-eeze quicksilver into a solid, 
while a moderate heat converts it into a vapour or heavy gas ; sulphm-, again, may be melted by heat 
into a liquid, and by greater heat brought also into the state of vapour. 
These elements combine together to form all the other substances, solid, liquid, or gaseous, wliich 
exist in or upon the earth. By the word combine is here meant, not merely a mixtm-e or conjunction 
of two elements, but a particular kind of union which it is the especial province of chemistry to 
explain, and which is called chemical combination. The natm-e of this may be most easily explained 
by an example : The air wliich we breathe is composed of oxygen and nitrogen, two simple elements, 
and these are merely in a state of mixtui-e, which may be compared to the state of sugar and water, 
when mixed together to form a syi'up, each ingredient still retaining its own essential characters 
unaltered. Water, on the other hand, is a chemical compound of the two elements, oxygen and 
hydrogen, two gases, which when thus combined, lose their peculiar characters for the time, and form 
a new substance different from both, — in like manner to the result produced when we pour an acid 
upon a metal, say nitric acid upon a copper halfpenny : the copper gradually disappears, the acid 
loses its corrosive acidity, and a beautiful blue liquid is produced as the result, which liquid, if wo 
evaporate it, yields crystals of the nitrate of copper, a substance differing in form, colour, textm'e, 
and all other qualities from both its elements. Such combinations, and on the contrary deco?njiositions 
or separations of compound substances into their elements, are going on unceasingly around us in 
every part of the creation. 
But these combinations are by no means formed at random ; they are subject to most strict and 
definite laws. Some elements will combine most readily with a certain number of others, not with 
all, or only under particular circumstances. In the changes which take place, we find the elements 
exhibiting preferences for particular of the rest, preferences which can even be reduced to a scale of 
degrees. This preference to unite with particular substances is what is called ajfinity ; and an element 
is said to have the greatest affinity for those others with which it forms the compound most difficult 
to decompose or separate into these component parts. And these affinities give rise to a wonderful 
diversity of interchanges, when the vai-ious compounds come in contact with each [other under various 
circumstances ; for if two compounds, consisting of elements which have but small affinity, meet 
together, they often exchange, and thereby produce, two new compounds. Thus, for example, if we 
have a compound of A and B, and another of C and D ; and the affinities between A and D, and 
between B and C, are stronger than those at present binding the elements together ; when the two 
compoands are brought to act upon each other, there will be a double decomposition, as it is called, 
and two new substances will be formed, one of A and D, the other of B and C, as the result of 
the interchange. Or, in a simpler case, if we take the compound A and B, and add the element C to 
it, the affinity between this and B being greater than that between A and B, C will displace A 
and form a new compound with B, while A is set free. As an example of the first, if we add the 
solution of nitrate of copper, obtained by dissolving a halfpenny in nitric acid, to a solution of 
carbonate of soda, since the affinity between nitric acid and soda is greater than that between the 
carbonic acid and the soda, they will leave their original compounds, and combine to form a nitrate of 
soda, while the carbonic acid will go to the copper to form a carbonate of copper, which, being insoluble 
in water, will fall to the bottom as a heavy powder. In the second case, if we add nitric acid alone 
