i88o.] 
On Water and Air. 
513 
pour into this longer tube the greater will be the amount of 
squeezing. Now, what Boyle wanted to ascertain is this : 
what is the relation between the pressure of the mercury 
brought to bear upon this air and the volume of the air ; 
and I will tell you what his result was. He found that 
when the pressure was doubled the volume was halved, and 
when the pressure was quadrupled the volume was reduced 
to one-quarter, so that as the pressure augmented the 
volume decreased in precisely the same proportion. Or, 
turning it into other language, instead of “ volume ” let me 
say “ density.” The density means the quantity of matter 
crowded into a certain space. If we use the term density 
Boyle’s law would be this, that the density of the air is 
diredlly proportional to the pressure exerted upon it. 
And now the mercury has risen in the long tube, and has 
reduced the volume of the air enclosed in the short arm to 
one-half its previous volume. If we now measure the 
height of the two columns, we shall find that the level of 
the mercury in the long arm stands very nearly at 30 inches 
above that in the closed portion. Thus the pressure of an 
additional atmosphere has reduced the bulk of the contained 
air to one-half. In this way, then, by doubling the pressure 
we halve the volume of the air, and by quadrupling it we 
should render the volume of the air one-fourth. In this way 
Boyle proved that the density of the air was exactly pro- 
portional to the pressure brought to bear upon it. I could 
not possibly dwell upon the subjeft were it not that I think 
that honour ought to be given where it is due ; and for a 
long series of generations, I may say, the discovery of this 
law has been ascribed to another person, — a most meritorious 
man, — a philosopher named Mariotte ; and you will find 
that in books, particularly in Continental works, this disco- 
very is always referred to as the law of Mariotte, but Boyle 
preceded Mariotte. He took the greatest care in his expe- 
riments, and he established with the utmost rigour the 
existence of this law anterior to Mariotte, and therefore the 
law ought undoubtedly to be called the law of Boyle instead 
of the law of Mariotte. Boyle, as I mentioned in our last 
ledture, had very clear notions regarding pressure of the 
atmosphere, and regarding what he described in beautiful, 
or, I may say, almost poetic language ; for men like Boyle 
have a strong poetic instindf, — that is to say, they can see 
resemblances between utterly remote things. They can 
pierce, as it were, into the nature of things, and see things 
resembling each other where the common eye sees no re- 
semblance whatever ; and thus, as I expressed in my last 
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