y 



t 6o DISCOVERY REPORTS 



until all the diatoms were resting on the silk. The glass tube D was then fitted on to the 



pipette end of the fine glass tube C and placed inside the cylinder. The apparatus was 



attached to the water supply by means of a piece of rubber tubing, 



and a slow stream allowed to pass through. Where the diatoms 



were very abundant, adequate portions of the filtrate were washed 



separately. 



Washing was allowed to continue for two or three hours. The 

 material was then removed from the silk, rinsed in distilled water 

 and stored in glass tubes in alcohol. 



The storage of phytoplankton samples in a solution of commercial 

 formalin is to be avoided, as it has been noticed that after a time 

 distinct changes in the hydrogen-ion concentration take place, par- 

 ticularly if the samples are placed in a position where they receive 

 direct sunlight. Atkins (1922*7, 6) has noted the effects of such 

 changes upon certain algal cells, and has recommended the use of 

 a permanently non-acid formalin for preserving calcareous speci- 

 mens (1922 c). Tests made on some of the earlier samples (1927) 

 dealt with in this work showed that the pH had been reduced to 

 approximately 5-0, while, although formalin prepared with borax 

 was used in later years, many of the 193 1 samples registered a pH 

 of 6-o. In view of this very considerable increase in acidity, the 

 weakly siliceous nature of some of the Antarctic diatoms, and the prolonged period of 

 storage, there is the possibility that some part of the original catch has not been 

 preserved. 



The usual method of cleaning diatoms, such as boiling with a mixture of pure 

 mineral acids, cannot be adopted with plankton samples, as some of the typical genera, 

 such as Chaetoceros and Rhizosolenia, are so weakly siliceous as not to survive so 

 vigorous a treatment. 



Further cleaning was effected after the diatoms had been placed upon the slide, and 

 consisted in washing with acetone and clove oil and removing the oil with xylol and 

 alcohol. All preliminary examinations were made in aqueous mounts. 



In order to make specific identifications of diatoms it was necessary to mount them in 

 a substance of a high refractive index. Owing to the extreme sensitiveness of plankton 

 forms to almost any degree of heat great care had to be exercised in selecting and using 

 the mounting medium. 



For the major part of the work balsam was used, although the mixture suggested by 

 Ghazzawi (1933) was often employed. This mixture consists of two parts of piperine 

 and one part of antimony tribromide finely ground together. An adequate quantity of 

 this mixture is added to a dry film of diatoms upon a glass slip. The slip is gently 

 warmed until the powder melts, when the cover glass is placed into position and gently 

 pressed down. 



The mount sets perfectly hard in about 2 min. and is of a canary yellow colour. 



Fig. 2. 



