1875.| and the Lower Animals. 65 
then known to be bodies composed of the same ultimate 
elements as our earth, governed by the same physical forces, 
and consisting, in the main, of a nucleus partly solid and 
partly liquid, surrounded by an atmosphere of gases and 
vapours. They were supposed to be semi-spiritual in their 
nature, formed of no one knew what, perfectly spherical in 
form, and ftee from such blemishes and imperfections as 
hills and valleys. The moon alone, being ‘‘ lowest” and 
nearest to the earth, had in consequence—as generally 
happens to those who indulge in ‘‘ questionable’ company 
—contracted certain stains and impurities. Readers not 
conversant with the history of astronomy will find difficulty 
in believing that such was the interpretation put upon those 
spots in the moon which the telescope resolves into moun- 
tains, chasms, and ravines. As for our earth, it was regarded 
as a place utterly vile and ‘‘impure,” the ‘‘ sink of the 
universe,’’—differing from the heavenly bodies in its nature, 
and contrasting with them in every respect. Those very 
features which conduce most to its beauty, and which alone 
render it habitable, were considered as proofs of its vast in- 
feriority. These absurdities were recognised as philosophic 
truths ; they were consecrated as articles of the true faith, 
and reigned for centuries unchallenged. Between earth and 
heaven there was a ‘great gulf.” But the progress of 
astronomy showed finally that the earth and the remaining 
planets were sister orbs, between which no such antithetical 
distinétion could be maintained. The “ great gulf” was 
found to be merely the fog and darkness of ignorance, and, 
as it has been beautifully expressed, “‘ earth was restored to 
her place in the heavens.” 
Turning from astronomy to chemistry, no antithesis in 
science was more firmly established a hundred years ago 
than that of acid and alkali, or, as we should now say, of 
acid and base. But in the progress of research it was found 
that one and the same body could fulfil either of these 
functions according to circumstances. ‘Thus alumina in 
contact with a body more basic than itself—such as potassa 
or soda—plays the part of an acid; but if brought together 
with a substance of a more decidedly acid charaéter—such 
as the sulphuric or nitric—it a¢ts as a base. Thus the old 
absolute view of acid and alkali as two classes, mutually 
exclusive and essentially antithetical, has passed away, and 
the terms have acquired a mere relative significance. In 
this case the change of view was fortunately not retarded 
by any extraneous complications, such as social or theological 
prepossessions. 
VOL. V. (N. S.) K 
