XXU INTRODUCTION. 



tincture of iodine causes the internal membrane to contract 

 upon the cell-contents, and converts these, from the golden yel- 

 low which they exhibit in some species, into a bright green ; and 

 that a weak solution of sulphuric acid, while it effects the same 

 contraction in the cell- wall, gives to the contents, which have 

 been previously treated with iodine, a dark brown hue. 



Alcohol, on the other hand, as in the case of vegetable cells 

 in general, dissolves the utricle and its contained endochrome, 

 or, at all events, entirely removes their colour, and leaves their 

 siliceous epiderm in a state of perfect transparency. It does 

 not, however, dissolve the envelope in which the frustules of the 

 frondose forms are imbedded, nor the filamentous stipes or 

 gelatinous cushions to which other species are attached. 



Section IV. 

 Movements of the Diatomace^. 



One of the most striking circumstances connected with the 

 living frustule, is the singular motion which most of the free 

 species exhibit. 



This motion is of a peculiar kind, being generally a series of 

 jerks, producing a rectilinear movement in one direction, and a 

 return, upon nearly the same path, after a few moments' pause, 

 by another series of isochronal impulsions. 



The movement is evidently of a mechanical nature, produced 

 by the operation of a force not depending upon the volition of the 

 living organisms. An obstacle in the path is not avoided, but 

 pushed aside ; or, if it be sufficient to avert the onward course 

 of the frustule, the latter is detained for a time equal to that 

 which it would have occupied in its forward progression, and 

 then retires from the impediment, as if it had accomplished its 

 full course. 



There is certainly no character of animality in the movement ; 

 and the observer familiar with the phaenomena of life in the ear- 

 lier stages of vegetable existence, is constrained to seek a counter- 

 part in the involuntary motions of the filaments of the Oscilla- 

 toriea, or of the gemmiparous spores of the Fuci and Confervce. 



