948 
THE VOYAGE OF H.M.S. CHALLENGER. 
CHAPTER XXII. 
Density of Sea Water — Composition of Ocean Water Salts — Geographical and Bathymetrical Distribution of Specific 
Gravity — Carbonic Acid, Nitrogen, and Oxygen in Sea Water — Discussion of Meteorological Observations as 
bearing on Oceanic Circulation. 
During the whole of the voyage very particular attention was paid to the determination 
of the density of the water and the study of its variations. In a freshwater lake the 
variations of density depend only on the temperature and pressure of the water, in 
the sea they depend also on the saline matter held in solution. The effects of change 
of temperature and of salinity on the density of water have formed the subject of many 
investigations, and have been ably and exhaustively reported on by Professor Dittmar 
in his Memoir . 1 The effect of pressure on sea water had been studied occasionally 
by Perkin, Airne, Regnault, and others. A number of experiments on the subject 
were made by Mr. Buchanan in the compression apparatus supplied to the ship, and 
also in instruments directly attached to the sounding line. Professor Tait has been, 
for some time, engaged in determining the compressibility of fresh and sea water at 
different temperatures and pressures, and the effect of pressure on the maximum density 
point of fresh water. Preliminary results have been published by him from time to time 
in the Proceedings of the Royal Society of Edinburgh. When these experiments are 
completed, as it is hoped they will soon be, when a new form of apparatus expressly 
devised for the purpose is delivered by the makers, the application to the Challenger 
observations will be made. It is impossible, without such preliminary work, to attack 
with any hope of success the problem of oceanic circulation. 
If the effect of change of salinity, of temperature, and of pressure on the density 
of the water be known, it is possible from a determination of its density at any 
temperature and pressure to deduce its salinity, and further to deduce the density of the 
same water at any other temperature and pressure. The method and instruments 
employed in the determination of the density of the water have been fully described in 
Mr. Buchanan’s Report. 2 The instrument used was a glass hydrometer weighing about 
160 grammes (see p. 109). Its stem was 100 millimetres long and 3 millimetres 
thick, and was divided into millimetres ; the volume of the divided portion of the stem 
was 0 '8 5 cubic centimetres. The weight of the instrument could be increased by attaching 
small weights to the top of the stem, and the variations in the volume of the instrument 
with changes of temperature were very carefully determined. When the hydrometer was 
1 Report on Researches into the Composition of Ocean-Water, Phys. Chem. Chall. Exp., part i., 1884. 
2 Report on the Specific Gravity of Samples of Ocean-Water, Phys. Chem. Chall. Exp., part ii., 1884, 
