CHAPTERS FOR STUDENTS. 
724 
quarter, or any other fraction of 16 grams of oxygen, or of 32 grams of 
sulphur; but the point to remember is, that the proportion between them must 
be preserved. To illustrate this. Suppose we wanted one-tenth of the Pharma¬ 
copoeia! quantity of any official preparation, for example, of pulvis antimonialis, 
we should take one-tenth of the prescribed quantity of each ingredient, that is, 
one-tenth of an ounce of oxide of antimony, and one-fifth of an ounce of phos¬ 
phate of lime. The proportion of the one ingredient to the other would be 
the same as usual, viz. 1 to 2, though the quantities really used would be 
different. . .... 
The proportions of the elements which are found to combine together in this 
unalterable manner are usually called the “ atomic’ * weights of the elements. 
The standard atoms possessing the absolute weight and volume adopted in 
these notes are the most easily applicable to purposes of calculation, and nume¬ 
rous examples of their utility will be found further on. 
The Atomic Theory was invented to explain the known laws of chemical com¬ 
bination. According to the atomic theory, all bodies are constituted of little 
particles, or atoms, so excessively small as to elude all attempts to see or 
examine them separately from each other. According to the theory, all the 
atoms of any one element are like each other, but different from those of any 
other element. Thus, for example, all the atoms of sulphur are supposed of the 
same size and weight, and all the atoms of hydrogen are of the same size and 
weight, but those of sulphur are 32 times heavier than those of hydrogen. 
When in chemistry we say that elements, or compound bodies, are built up of 
atoms, it is not intended by this to imply that the atoms are really and truly 
indivisible in the absolute sense of the word. We only suppose each atom to 
be a certain small mass which is never broken or divided by making it combine 
with other atoms, but we do not know w'hether it is itself constituted of smaller 
particles or not. Indeed, in chemistry, the term u atom has gradually ac¬ 
quired a modified signification ; it is usually now taken to mean any one of the 
constituents of a compound which is capable of retaining, under certain condi¬ 
tions, a certain degree of independence,—so that it is actually often applied in 
connection with bodies which, instead of being simple, are composed of more 
than one elementary atom. 
When any two symbols are placed side by side, this juxtaposition shows that 
the elements they represent are chemically combined together. lor instance, 
H 2 0, llCl, N H 3 . C 2 H 6 0 are respectively the “ formulae ” of the chemical 
compounds water, hydrochloric acid, ammonia, and alcohol. The formulae of 
compounds have a meaning analogous to that of the elementary symbols. They 
represent certain fixed weights, which are simply the sum of the weights repre¬ 
sented by the symbols separately. 
H 2 0 stands for (2 + 16=) 18 grains of water. 
HC1 stands for (1 -+-35-5=) 36’5 grains of hydrochloric acid. 
NHj stands for (14+3=) 17 grains of ammonia. 
C 2 H 6 0 stands for (12x2 + 6 + 16=) 46 grains of alcohol. 
Moreover, the weights of all compounds represented by such formulae, when 
converted into vapour, brought to the same temperature, and measured under 
the same pressure,f are again found to occupy the same cubic space. I his 
space is double of that which has been given as the volume of the gram-atoms 
of the elements. Accordingly,— 
* Atom, that which cannot be cut; from a and refxua. A list of atomic weights will be 
found at the end of the British Pharmacopoeia, 1867. 
f The temperature and atmospheric pressure at which such measurements are made are 
expressed by 0° C. and 760 millimetres (29’92 inches) of mercury in the barometer. 
