200 



Messrs. Carpenter and Jeffreys on 



[Dec. 8, 



97. Our attention was drawn to this point, in the first instance, by 

 finding that the bottom-water brought up by the Water-bottle was nearly 

 always turbid, and that this turbidity was with difficulty removed by fil- 

 tration. The bottom-water brought up from sandy or gravelly bottoms 

 is always clear ; and though that which was brought up from the area 

 covered by the " Atlantic mud " was often turbid, it was readily cleared 

 by passing through filtering-paper, the deposit on which was found to 

 consist of very minute Globigerince, which had been apparently floating 

 in the stratum immediately above the Sea-bed. As the clearing of the 

 Mediterranean water was requisite for our Chlorine-determinations, it was 

 passed twice or thrice through the filter, and the solid matter left upon 

 the paper consisted entirely of Inorganic particles of extreme minuteness. 

 Now it is a fact well known to Chemists and Physicists, that the length of 

 time required for the deposit of a precipitate increases with the fineness 

 of the division of its particles, notwithstanding that the material of which 

 they are composed may be of very high Specific Gravity. Thus it was 

 shown by Faraday that precipitates of Gold may not subside for a month ; 

 and Mr. Babbage has calculated that, in the case of lighter substances, a 

 period of hundreds of years may be required for the gravitation of very 

 finely divided particles through a considerable mass of fluid. 



98. Taking into account, therefore, that the deep waters of the Medi- 

 terranean are not only cut off from the General Oceanic Circulation, but 

 that they are almost entirely destitute of vertical circulation amongst 

 themselves (§ 95), it may be fairly considered that the perceptible turbi- 

 dity of the bottom-water is due to the imperceptible diffusion of the same 

 finely divided matter throughout the entire mass of superincumbent water. 

 And that this is really the case, is shown by two different methods of 

 proof. We learned from the Engineer of the Peninsular and Oriental 

 Company's Steam-ship by which we proceeded to join the 'Porcupine' at 

 Gibraltar, that the deposit removed from the boilers after working in the 

 Mediterranean differs from that left by Atlantic water, not only in its 

 larger proportion of salt, but in having a very finely divided mud diffused 

 through it, which is, of course, derived from the evaporation of surface- 

 water. The result of this large-scale experiment harmonizes exactly with 

 that of Prof. TyndalPs examination of a small sample of the surface-water 

 of the Mediterranean by the Electric light ; for he found it to be highly 

 charged with minute particles in suspension, as is also the water of the 

 Lake of Geneva. And he has further shown that it is in each case to the 

 presence of these particles that we are to attribute the peculiar intensity 

 of the blue colour by which both these waters are characterized*. 



* See 'Nature,' Oct. 18, 1870. — We may take leave to mention that the same idea 

 of the agency of the suspended particles in intensifying the blue colour of the water 

 had previously occurred to ourselves, and had been made the subject of conversation 

 on our voyage home, the probable community of the source of the suspended particles 

 in the Mediterranean and the Lake of Geneva respectively having especially presented 



