The Evolution of Scientific Knowledge. 
[July, 
424 
earth’s crust to the history of our knowledge concerning the 
constitution of the matter of which that crust is composed, 
we shall obtain further exemplification of the Law of Evo. 
lution. Among the early Greeks we find prevalent the 
dodtrine of the four elements, Earth, Water, Air or Steam, 
and Fire, of which all bodies were supposed to be composed. 
But as Dr. Roscoe points out, these terms denoted rather 
general properties than particular substances. Thus earth 
implied the properties of dryness and coldness ; water, those 
of coldness and wetness ; air or steam, wetness and heat ; 
fire dryness and heat. All matter was, indeed, supposed to 
be if one kind, the variety which we observe being accounted 
for bv the greater or less abundance of these four conditions. 
Here then, we have a theory which is sufficiently vague, 
general, and indefinite. Somewhat similar, but more ad- 
vanced, are the views of the Arabian alchemist, Geber, 
according to whom “ the essential differences between the 
metals are due to the preponderance of one of two prin- 
ciples, mercury and sulphur ; of which all the metals, are 
composed.” According to him “ the noble metals contained 
a very pure mercury, and were therefore unalterable by heat, 
whilst the base metals contained so much sulphur that they 
lost their metallic properties in the , T ° ? 1 \ ese , t ; w f 
principles Basil Valentine subsequently added a third, which 
he called Salt : while Lemery, calling these the aftive pnn- 
cinles added two more, which he termed passive, namely, 
water or phlegm and earth. The first philosopher, however, 
who seems to have grasped the idea of the distinction be- 
tween an elementary and a compound body, was Kobe rt 
Bovle * and this conception cannot but be regarded as an 
immense advance in descriptive chemistry. But the theory 
which had the most marked influence on the history ot 
chemistry in the seventeenth century was that which was 
started by Becher and developed by Stahl, and which is 
known as the Phlogistic theory. When magnesium wire is 
burnt in the air a white solid is formed. We now know 
that this is the result of the combination of the metal with 
the oxygen of the air; but, according to the Phlogistic 
theory, this white substance results from the fatf: that the 
metal has lost a “ combustible principle 5 \ which Stahl 
termed phlogiston. It was upon this hypothesis that Black, 
Priestley, and Cavendish— the founders of ex ^!\ ch 5. mi , ca J 
science — worked, and it was their labours which afforded 
the observations which resulted in the complete overthrow 
of that theory. For the Phlogistic hypothesis, like the geo- 
centric conception of the Solar System, was ere long found 
