Mr. Thompson on an Alga which colours Bally drain Lake. 79 



Sept. 27. — I went out this morning (which was beautiful 

 and with bright sunshine) soon after eight o^clock to observe 

 this plant further^ and from a greater display of it upon the 

 surface than yesterday, considered that the heat of the sun 

 might have attracted it thither. I had previously observed, 

 that in perfectly calm weather, it, like the Oscillatoria ceruges- 

 cens, &c., is disposed to ascend to the surface ; it was now evi- 

 dently beginning to assume the graceful and attractive forms 

 remarked on the 16th. By fixing a phial to my net and skim- 

 ming the greenish surface with it, I ascertained to a certainty 

 that this surface plant was of the species under consideration. 

 In addition to what was mentioned yesterday of the water of 

 the lake having lost some of its opacity, it may be stated, that 

 where a foot in depth it now in some places is perfectly clear, 

 but in others where it is from two to three feet, it appears 

 when the sun shines upon it of a very pale blue; whether this 

 be an optical deception or be owing to the plant in progress 

 of decomposition, I cannot presume to state. 



Oct. 7« — This plant and the Aphanizomenon (hereafter to 

 be particularly noticed), both of which were obtained on the 

 27th Sept., have now entirely disappeared from the lake, the 

 water throughout its depth as well as at the surface being clear 

 and pure : since the day last named there has been no change 

 of weather to produce this effect, the days having been uni- 

 formly warm and fine, and the nights with very little frost for 

 this advanced period of the year. 



The following additional notes were made in 1839. 



July 3. — I was rowed all over the lake, and observed the 

 alga dispersed throughout the entire water, but rather spa- 

 ringly, and not to such an extent as to tinge or conceal the 

 bottom in any place ; it appeared like metallic points where- 

 ever the sun shone upon it ; for some weeks it has been ob- 

 served, and was first noticed about the 6th of June. On the 

 present occasion I had the pleasure of being accompanied by 

 P. J. Selby, Esq. and the Rev. Edward Bigge, of Merton Col- 

 lege, Oxford. 



Sept. 23. — The lake was quite clear, and in a few places the 

 remains only of the blueish masses which indicate the disap- 

 pearance of the plant for the season were visible, and these 



