SIR DAVID BREWSTER ON THE COLOURS OF THE SOAP-BUBBLE. 



501 



hitherto described, that a more careful study of them may enable us to form some 

 idea of their structure, and of the matter of which they are formed. A very few 

 of their numerous and ever-changing forms are shown in the annexed diagram, 

 and in Figs. 14 and 15. Their outline is generally well defined; and when they 



are ascending through the colouring matter of any band they displace it, raising a 

 little its tint at their margin, as at M. The breadth of their head seldom exceeds 

 the thirtieth of an inch, and it increases with the size of the film. They carry in 

 their heads, and also in their tails, colouring matter of various tints and orders ; 

 and when borne to the top of the films, as at A, by the lighter matter m of the first 

 orders, they leave it in its place, and return with their heavier freight n to deposit 

 it in the bands to which it belongs. Single tadpoles are often united together in 

 the most capricious manner, both laterally and by their tails ; and when the 

 whole film is covered with them and without bands, they will lay themselves 

 together, so that a band of a particular colour will be formed by their similarly- 

 coloured tails, and another adjacent band by their similarly-coloured heads. 



When a plane film is slightly inclined to the horizon, a number of colourless 

 circular portions and colourless tadpoles will at first move over its surface, rising 

 to its apex, chiefly round its circumference. These tadpoles gradually show 

 colours of different orders, which are carried to their appropriate bands. A large 

 portion of this colouring matter is frequently carried up the margin of the film to 

 its apex, where the black band must be formed ; but, as it cannot remain there, 

 it moves slowly downward, reinforcing the bands to which it belongs, and leaving 

 the apex for the formation of the black band. 



When the black band is nearly as large as in Fig. 5, and there is colouring 

 matter above it, it will cut its way through the black mass, dividing it into three 

 or four portions, separated by narrow channels, through which the colouring 

 matter moves down in small specks to join their proper bands. When this is 

 done, the three or four portions of the black band unite, and the band expands 

 itself over the whole film, pushing before it, in striped currents or in circular 

 portions, the higher orders of colouring matter, which returns to its original 



