NARRATIVE OF THE CRUISE. 
993 
These observations on the rain water of equatorial regions prove that it is highly 
charged with organic matter of animal or vegetable origin which is readily oxidised and 
absorbs for this purpose the oxygen dissolved in the water. When this water falls upon 
land the whole of the organic matter is arrested by the soil and utilised in the support 
of the dense vegetation which characterises these regions. Purified by the most efficient 
form of filtration, through clean soil, the water joins the stream free from all liability 
to putrefaction. When it falls directly on the sea, it remains in the surface layer where 
it is exposed to all the power of the sun’s light and heat. In these regions calms prevail, 
and oxidation may easily go on too rapidly for the immediate replacement of the 
removed oxygen. The greatest, deficiency of oxygen in these regions only amounts to 
one per cent. In a liquid in a state so near to complete saturation, renewals are only 
effected slowly. 
In distilled or rain water it is admissible to compare the carbonic acid with the 
oxygen, which is deficient, and to consider them to some extent as mutually inter- 
dependent. In sea water no such comparison can be made, because the carbonic acid is 
present in it in such abundance, and is retained in it by an affinity much stronger than 
that caused by absorption. While the amount which is present in virtue of absorption from 
the atmosphere cannot exceed a cubic centimetre and a half, the amount actually present 
which can be eliminated by boiling with chloride of barium solution rarely falls below 
10 c.c. even in the warmest regions and at the surface, while in the cold water of the 
bottom of the oceans it may amount to as much as 40 c.c. per litre. According to 
Professor Dittmar’s experiments, sea water of the lowest temperature met with cannot 
contain more than 8T8 c.c. of oxygen per litre, hence the maximum amount of carbonic 
acid which can be due to the consumption of oxygen by carbon after the water has left the 
surface is' 8T8 c.c. It was rare to meet with water which had lost one-half of its oxygen, 
while a loss of one-third was not uncommon. Hence not more than from 3 to 4 c.c. of 
the carbonic acid present in a water from the bottom or intermediate depths is likely 
to have been produced at the expense of the oxygen which it held dissolved. The 
carbonic acid eliminated by distillation from such waters varies from 15 to 40 c.c., it is 
therefore altogether impossible to trace in it the oxygen which is deficient. 
The amount of carbonic acid present in the gas tubes along with the oxygen and nitro- 
gen has been extracted by boiling in vacuo, and therefore deserves attention. In the 
gases from surface waters it averages about 15 per cent, of the total gas, there being very 
few cases even in the coldest waters where it exceeds 20 per cent. In bottom waters it 
"averages 27 per cent., in several cases it exceeds 40, and in one case reached 50 per cent. 
In intermediate waters from 300 fathoms and greater depths, it averages 25 per cent., 
the maximum being 40 per cent. The amount of carbonic acid extracted in this way 
from bottom and intermediate waters is therefore much greater than can be taken from 
surface water, although the average amount of loosely bound or free carbonic acid as 
