330 . PHYSICAL GEOGRAPHY. 



enormous quantities, through an extent of more than 16 of latitude. Speculating upon 

 its cause, the distinguished chemist remarks : "It appears to me, that there are only 

 two sources to which it can with any probability be referred, namely, submarine volcanic 

 action, in which case its evolution might be considered direct or primary ; and the reac- 

 tion of vegetable upon the saline contents of the water, in which case it would be secondary. 

 The probability of a volcanic origin is, I think, small, from the absence, I believe, of any 

 other indications of volcanic action, and from the great extent of the coast along which 

 it has been traced. What is known of the action of vegetable matter upon the sulphates, 

 and the immense quantities of vegetable matter which must be brought by the rivers 

 within the influence of the saline matter of the sea, renders, on the contrary, the second 

 origin extremely probable. Decaying vegetable matter abstracts the oxygen from 

 the sulphate of soda, and a sulphuret of sodium is formed. This again, acting upon water, 

 decomposes it, and sulphuretted hydrogen is one of the products of the decomposition." 

 There can be no doubt but that extensive banks of vegetable detritus have been formed 

 at the mouths of the rivers of -the west coast of Africa. They flow from an interior 

 country rich with the luxuriance of a tropical vegetation, and roll along in immense floods 

 in the rainy season, bringing down masses of decaying foliage into the ocean ; and what 

 renders the preceding explanation the more certain, is, that those inlets along the coast of 

 India, where the bottom contains carbonaceous matter, display the evolution of the same 

 gas. With this circumstance, the unhealthiness of the West African stations, which has 

 obtained for Sierra Leone the title of " the white man's grave," is intimately connected, for 

 it has been experimentally found that so small a mixture as a fifteen hundredth part of 

 sulphuretted hydrogen in the atmosphere acts as a direct poison upon small animals. The 

 mangrove swamps in all parts of the world are notoriously unhealthy, arising from the 

 tree requiring salt water for its growth, the sulphates of which are decomposed by the 

 decaying vegetable matter which is annually furnished. 



We are familiar with the phrases, the blue sea, and the green sea, with reference to the 

 colour of its waters. Its usual colour is a bluish green, of a darker tint at a distance 

 from land, and clearer towards the coasts, but subject to remarkable changes, which are 

 not well understood. According to Scoresby, the hue of the Greenland Sea varies from 

 ultra-marine blue to olive green, and from the purest transparency to the greatest opacity. 

 The prevailing blue colour may be ascribed to the greater refrangibility of the blue rays 

 of light, which, by reason of that property, pass in the greatest quantity through the 

 water. Humboldt observes of the cyanometer, an instrument for measuring the intensity 

 of colour, that when, instead of directing the apparatus to a large extent of open sea, the 

 observer fixes his eyes on a small part of its surface, viewed through a narrow aperture, 

 the water appears of a rich ultra-marine colour. Towards evening again, when the 

 edges of the waves, as the sun shines upon them, are of an emerald green, the surface of 

 the shaded side reflects a purple hue. Nothing, he states, is more striking than the rapid 

 changes which the ocean undergoes beneath a serene sky, where no variations whatever 

 are to be perceived in the atmosphere. In the midst of the tropical deep the water 

 passes from an indigo blue to the deepest green, and from this to a slate grey, without 

 any apparent influence from the azure of the sky or the colour of the clouds. In general 

 the sea between the tropics is of a more intense and purer azure than in high latitudes. 

 The ocean often remains blue when, in fine weather, the greater part of the sky is 

 covered with light and floating fleecy clouds. Humboldt concludes his observations upon 

 the tints of the ocean, and its changes from blue to green, with some general remarks, 

 the substance of which is embodied in the succeeding paragraphs. Whatever relates to 

 the colour of water is extremely problematic. The green tint of the snow waters 

 that flow from the Alpine glaciers, which contain very little air in solution, might induce 



