In the harbour of Gibraltar, on the morning of our departure, I resumed a series of observations on the colour of the sea. On the way out a number of specimens had been collected, with a view to subsequent examination. But the bottles were claret bottles, of doubtful purity. At Gibraltar, therefore, I purchased fifteen white glass bottles, with ground glass stoppers, and at Cadiz, thanks to the friendly guidance of Mr. Cameron, I secured a dozen more. These seven-and-twenty bottles were filled with water, taken at different places between Oran and Spithead. And here let me express my warmest acknowledgments to Captain Henderson, the commander of H.M.S. "Urgent," who aided me in my observations in every possible way. Indeed, my thanks are due to all the officers for their unfailing courtesy and help. The captain placed at my disposal his own coxswain, an intelligent fellow named Thorogood, who skilfully attached a cord to each bottle, weighted it with lead, cast it into the sea, and, after three successive rinsings, filled it under my own eyes. The contact of jugs, buckets, or other vessels was thus avoided; and even the necessity of pouring out the water, afterwards, through the dirty London air. The mode of examination applied to these bottles has been already described. [Footnote: On Dust and Disease, p. 168.] The liquid is illuminated by a powerfully condensed beam, its condition being revealed through the light scattered by its suspended particles. 'Care is taken to defend the eye from the access of all other light, and, thus defended, it becomes an organ of inconceivable delicacy.' Were water of uniform density perfectly free from suspended matter, it would, in my opinion, scatter no light at all. The track of a luminous beam could not be seen in such water. But 'an amount of impurity so infinitesimal as to be scarcely expressible in numbers, and the individual particles of which are so small as wholly to elude the microscope, may, when examined by the method alluded to, produce not only sensible, but striking, effects upon the eye.' The results of the examination of nineteen bottles filled at various places between Gibraltar and Spithead, are here tabulated: No. Locality Colour of Sea Appearance in Luminous beam 1 Gibraltar Harbour Green Thick with fine particles 2 Two miles Clearer green Thick with very from Gibraltar fine particles 3 Off Cabreta Point Bright green Still thick, but less so 4 Off Cabreta Point Black-indigo Much less thick, very pure 5 Off Tarifa Undecided Thicker than No. 4 6 Beyond Tarifa Cobalt-blue Much purer than No. 5 7 Twelve miles Yellow-green Very thick from Cadiz 8 Cadiz Harbour Yellow-green Exceedingly thick 9 Fourteen miles Yellow-green Thick, but less so from Cadiz