398 PROCEEDINGS OF THE SOCIETT. 



Mr. Jackson's gas theory was one which he had not met with before, 

 though he was not sure that the idea was altogether new. 



Mr. Karop suggested that it might be possible to apply some 

 harmless test or indicator of free oxygen, although he confessed he did 

 not know of any. 



Mr. W. J. Dibdin said that the evolution of free oxygen might be 

 easily detected by chemical reaction. In the presence of alkaline 

 hyposulphite of soda, indigo was completely decolorised, but in the 

 presence of free oxygen the indigo at once resumed its blue colour ; the 

 reaction took place with great rapidity, and testing for oxygen in this 

 way was perfectly feasible, provided the diatoms were not injured by the 

 medium. 



The President thought there could be no doubt as to the evolution 

 of oxygen by chlorophyll-containing vegetable organisms : this was 

 admitted by everybody ; but whether the emission of the gas was me- 

 chanically efficient in producing the motion was another matter. The 

 somewhat crude objection might be made that so many other aquatic 

 organisms likewise evolved hydrogen, but in the case of diatoms this 

 process might conceivably be regulated by the peculiarities of the silicious 

 cell-wall so as to modify the mechanical effect. He should very much 

 like to see the sham diatoms which had been mentioned in the paper 

 as having been constructed by the author. 



Mr. Michael said the number of people who were specially diatomists, 

 in the true sense of the term, was unfortunately becoming extremely 

 small, and it was, therefore, not easy to obtain opinions of value con- 

 cerning them, but it appeared to him that it was rather difficult to 

 understand how the escape of free oxygen was competent to account 

 for some of the peculiar movements observed amongst diatoms, such 

 as vibratory movements ; or how it could account for the opening and 

 shutting of the fan-like forms which grew on a stalk. It was, how- 

 ever, possible that it might be effected in some way by the position 

 of the channels through which the oxygen had to pass in escaping. 



The thanks of the Society were, on the motion of the President, 

 unanimously voted to Mr. Jackson for his communication. 



The President said they had upon the table an interesting exhibition 

 of Oribatidas, and as Mr. Michael, who had originally presented the 

 specimens to the Society, was present, he would no doubt be able to say 

 a few words in reference to them. 



Mr. A. D. Michael said he had been rather surprised to hear that 

 there was to be an exhibition of slides which he had presented to the 

 Society about twenty years ago. He could no doubt say something on 

 the subject, but it was not so easy to say something new, as all he knew 

 about the subject was contained in his papers read before this Society 

 and his book published by the Ray Society. It was a long time since 

 that book was written, and seeing that it referred only to the specimens 

 in one man's collection, he then thought that there would in course 

 of time be many additions to the list of British Oribatidre, but up to 

 the present only one specimen had been added to it. The Oribatidaa 

 in their adult form were rather beetle-like creatures and not specially 



