92 
This is now very satisfactorily explained by 
another really great man of science—a man 
whose work for chemistry is quite as funda- 
mental as his work for physics—I refer, of 
course, to Sir J. J. Thomson. 
He has shown that whether or not precipi- 
tates are formed is dependent upon the pres- 
ence or absence of appreciable numbers of 
charged parts or ions. Arsenic sulphide is 
precipitated from the solution of the chloride 
because the hydrocloriec acid set free by the 
action of the hydrogen sulphide is strongly 
ionized. On the other hand, arsenic sulphide 
is not precipitated from the solution of the 
oxide, because no strongly dissociated sub- 
stance is formed as the result of the reaction, 
and neither arsenic oxide nor hydrogen sul- 
phide is strongly dissociated. 
But Thomson does not stop with showing 
that ions or charged parts are necessary for 
precipitation. It was shown by Burton, work- 
ing in Thomson’s laboratory, why, or at least 
how, this is the case. Space will not allow me 
to go into this in detail. Suffice it to say here 
that the colloidally suspended particles are 
charged electrically, and for any given col- 
loid all of the particles are charged with the 
same sign. These electrical repulsions work 
counter to surface-tension, which acts so as 
to draw the particles into the smallest surface 
for a given mass—to draw the colloidally sus- 
pended particles into lumps as in an ordinary 
“precipitate. When ions are present these 
electrically neutralize the charges upon the 
colloidal particles and allow surface-tension 
to produce its full effect. 
That ions are necessary and suflicient to 
effect precipitation, can readily be shown by 
adding almost any electrolyte to the colloid- 
ally suspended particles of arsenic sulphide, 
obtained by treating the oxide with hydrogen 
sulphide. A precipitate is formed at once. 
This work places the whole subject of pre- 
cipitation, for the first time, upon a scientific 
basis, and while it can not be presented fully 
to a beginner, I see no reason why it should 
not be judiciously taught to a student in his 
second year of chemistry, z. e., when he is 
SCIENCE 
[N.S. Vou. XXXV. No. 890 
studying 
alysis. 
Then arise some of the most fundamental 
problems. What is a chemical atom? If 
made up of parts what are these parts, and 
how are they arranged within the atom? 
How does one chemical atom differ from 
another chemical atom? Are the chemical 
atoms stable? 
These matters must all be taught the stu- 
dent of chemistry and the question is when? 
They can not of course all be presented fully 
to what we ordinarily mean by a beginner, but 
I can see no reason why they can not be pre- 
sented, in an elementary manner of course, at 
the proper places, even in the first year’s work 
in chemistry, unless we are wedded to the 
dogma that chemistry must be made easy in 
order that it may be popular. 
We can certainly no longer teach that the 
chemical atom is an “ultimate unit” in the 
light of the recent work of Thomson. We 
know that it is made up of parts, and further- 
more, we have some idea how these parts are 
arranged in two dimensions in space in a sec- 
tion through the atom. We have very good 
reason to believe that most, if not all of the 
differences between the atoms of the various 
qualitative and quantitative an- 
chemical elements are a function of the num- 
ber, arrangement, and possibly the velocities 
of the electrons composing the atoms. And 
why not, in a common-sense manner, tell the 
student of chemistry so, even in the com- 
paratively early stages of his work? 
Indeed, I think it is far simpler to teach 
this fundamental connection between the ele- 
ments, than to have the beginner look upon 
the eighty or more elementary substances 
as so many discrete, disconnected, and funda- 
mentally unrelated kinds of matter—to say 
nothing of it being true; and in the teaching 
of science I think truth is even more impor- 
tant than simplicity. 
And again, take the question of the stabzl- 
ity of the chemical atom. The stable atom of 
the past is now hardly more than historically 
interesting. The work of the Curies and espe- 
cially of Rutherford, on radioactive sub- 
