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POPULAR SCIENCE REVIEW. 
the change of water. (See “ Crystal Palace Aquarium Hand- 
hook,” 1 875, p. 7.) But now, having got to think more broadly, 
I recognise this, not as a change of water in the sense of its 
being lost, but merely as a change of position from a house in 
Edinburgh to the sea, and back again. That is to say, the 
water he dismissed from his jars went into a gutter in a street, 
or into a sewer below it, and found its way by gravitation into 
the ocean again. Or, if it were poured on the ground into 
which it soaked, it found its way back to the sea by an infinitely 
more circuitous route. But, had Dalyell been more of a general 
philosophical thinker as well as a naturalist, he would have 
saved himself this very great amount of cost and trouble. Had 
he but reflected on that which was then known, namely, that 
water — both sea-water and fresh water — is practically inde- 
structible, and that any decaying organic matter, animal or 
vegetable, or both mixed, can be got rid of, and the water be 
left pure, then he would have saved his servants their weary 
walks of more than as far, in their aggregation, as twice round 
the world, nearly. 
In the ocean of course various animals and plants are inces- 
santly dying in large numbers, and their decomposing remains 
are prevented from permanently poisoning the water in which 
other animals live and breathe, by the incessant motion to 
which the sea is subjected, and this motion brings the water 
into purifying contact with the atmospheric air which every- 
where exists. It is this air, or rather the oxygen in it, which 
the water takes Up in greater quantity than the nitrogen, which 
is another and larger component of the atmosphere, which is 
the source of purification alluded to, the water being merely a 
medium or a vehicle for the exhibition of the oxygen. In 
addition to this, vegetation grows by the action of light, 
and decomposes the poisonous carbonic acid gas evolved by 
the breathing of animals, the carbon being used to form the 
woody substance of the plants, and the residual oxygen being 
liberated for the use and benefit of the animals. Thus the 
ocean, and rivers, and lakes, and all other waters in nature, of 
varying degrees of freshness and saltness, by motion and vege- 
tation, both originating from the sun, are maintained suffi- 
ciently pure and respirable. 
These operations were going on almost at Dalyell’s door, yet 
he did not learn to apply them to practice, as he might have 
done. AVhat he did was this : He fed the animals in his jars 
on mussel flesh, which is easily diffusible in water, and which 
quickly makes it milky; and this, with the absence of growing 
vegetation, and the breathing and other emanations of the 
animals, soon caused the water to become offensive in appear- 
ance and in smell. So he threw it away. But the very act of 
