174 FKAGMENTS OF SCIENCE. 



others, the water flashing with metallic lustre. The 

 screw-blades in this case played the part of the dinner- 

 plate in the former case, and there were other instances 

 of a similar kind. The white bellies of porpoises 

 showed the green hue, varying in intensity as the 

 creatures swung to and fro between the surface and 

 the deeper water. Foam, at a certain depth below the 

 surface, was also green. In a rough sea the light which 

 penetrated the summit of a wave sometimes reached the 

 eye, a beautiful green cap being thus placed upon the 

 wave, even in indigo water. 



But how is this colour to be connected with the sus- 

 pended particles ? Thus. Take the dinner-plate which 

 showed so brilliant a green when thrown into indigo 

 water. Suppose it to diminish in size, until it reaches 

 an almost microscopic magnitude. It would still behave 

 substantially as the larger plate, sending to the eye its 

 modicum of green light. If the plate, instead of being 

 a large coherent mass, were ground to a powder suffi- 

 ciently fine, and in this condition diffused through the 

 clear sea- water, it would also send green light to the eye. 

 In fact, the suspended particles which the home exami- 

 nation reveals, act in all essential particulars like the 

 plate, or like the screw-blades, or like the foam, or like 

 the bellies of the porpoises. Thus I think the green- 

 ness of the sea is physically connected with the matter 

 which it holds in suspension. 



We reached Portsmouth on January 5, 1871. Then 

 ended a voyage which, though its main object was not 

 realised, has left behind it pleasant memories, both of 

 the aspects of nature and the kindliness of men. 



