Robert Edmond Grant to Faraday   15 March 1839

To Dr. Faraday

My Dear Sir

The green colour of the water left with you by Sir J. South, appears to be entirely dependent on the common fresh-water bacillarian animalcule, Desmidium Swarzii of Agardh¹ which has been so often figured and described by English & foreign botanists as a confera.

In the specimens which I have examined the water has presented no other forms of organic beings excepting these long, fine, green coloured, moving filaments, which are quite free, and generally collect near the surface of the fluid. I send you a specimen dried between glass and mica where, with the 40th of an inch power, you may easily see the internal structure of the component animalcules of each filament, as they are represented in the accompanying large diagram copied from Ehrenberg's figure (Tab. x. fig. 8) of this animalcule².

It appears to abound in the fresh waters of most of the temperate parts of Europe - England, Sweden, Denmark, Germany. I have seen, in these specimens, both the lateral bending motions and the slow creeping longitudinal movements of these singular aggregate animals.

This animal belongs to a great family in which the skeleton is most generally a thin external shell of silica, but in the Desmidium the external covering is flexible like parchment, and as you possess so large a portion of this matter I think it a favourable opportunity for determining whether a particle of silica can be detected in its organisation.

I remain My Dear Sir | Yours most truly | Robert E. Grant

10 Seymour Place North | Euston Square | 15 March 1839.

Address: Dr. Faraday | Royal Institution of Gr. Br.