566 Reviews. | July, 
giving a systematic account of the combinations of the elements, it 
might be thought an important part of the duties of the writer to dis- 
criminate clearly between those substances that have been separated and 
analyzed, those whose existence is rendered probable either by experi- 
mental evidence, falling short of demonstration, or by considerations 
of analogy, and those in favour of whose existence there is no presump- 
tion whatever. The reader of most elementary works on ‘chemistry 
will look in vain for this distinction. He is presented with the names 
and formule of a host of imaginary substances, many of which are so 
entirely with. ut analogues that no chemist would dream of attempting 
their preparation. Dr. Apjohn gives three lists, and similar lists may 
be found in most chemical text-books, of the oxides of Sulphur, 
Phosphorus, and Carbon. The first list consists of seven oxides, the 
second of four, the third of six. Chemists are actually acquainted 
with two oxides of Sulphur, two, or perhaps three, oxides of Phos- 
phorus, and two oxides of Carbon. Of the remaining oxides it is said 
that they “ exist only in combination.” This is one of those treacherous 
phrases of which chemistry were well rid. In this sense all bodies 
exist whose formule can be made by combining the symbols which 
compose the formula of an actually existing body. For example, the 
well-known salt hyposulphite of sodium has the constitution expressed, 
on the old notation, by the formula Na S°O*. Hence, on this principle, 
we mnay infer the existence of substances having the following for- 
mule :—Na 8, NaS’, Na O, Na 0”, Na O%, SO, SO., SO*, S70, S*O?, 
S’0*, Na SO, Na SO,, NaSO,, Na S°O, Na S?O?; of these substances 
those whose formule are printed in italics “ exist only in combination.” 
The reason why the formula of one of these imaginary bodies figures 
as that of “a known oxide of sulphur” (p. 256), is the traditional 
acceptance of the dualistic hypothesis, according to which every salt 
containing oxygen consists of a metallic oxide and an acid anhydride. 
Now we are far from saying that this oxide of sulphur may not here- 
after be made, or that there is no argument from analogy in support 
of this anticipation. Only the same may be said of nearly every one 
of the hypothetical bodies whose formule we have written down. The 
discovery of the teroxide of sodium, of the protoxide and suboxide of 
sulphur, of the sodium salts intermediate between the sulphide and 
sulphite, appears at least as probable. These indications of what we 
may hope to realize, drawn from the analogy of existing compounds, 
are the clue which must guide the chemical investigator ; but to set 
before a beginner the names and formule of certain substances selected 
on a particular hypothesis,—one out of many that have been formed,— 
from among hundreds of others, equally possible, equally unknown, is 
a course in the highest degree arbitrary and misleading. 
We wish to repeat, in conclusion, that our object has been to call 
attention to various points in which, as we venture to think, the tra- 
ditional teaching of chemistry is in fault. We have thus been led to 
notice chiefly those parts of Dr. Apjohn’s manual which illustrate the 
objections we have advanced, and have left without comment, as beside 
our purpose, the large amount of well-arranged information which it 
