CENTRIFUGE IN POND-LIFE WORK. 



245 



sometimes, owing to the great abundance of the organisms in 

 particular samples, he found it advisable to take no more than 

 o CO. for centrifuging. In my own experiments, working mainly 

 with water from small lakes and ponds, I have even reduced the 

 amount still further ; for I find that with tubes holding only 

 1| c.c. I can usually obtain sufficient deposit to permit of 

 qualitative investigation, though not perhaps sufficient in all 

 cases for accurate quantitative w^ork. The fact that such small 

 qviantities of water scarcely ever fail to yield appreciable results 

 points to the conclusion that the very minute organisms, ranging 

 in size from about yoVo ^^^- ^^own to the smallest bacteria, are 

 practically always present in very considerable numbers in the 



Plan and elevation of " haematocrit " head adapted to carry small vase- 

 shaped tubes holding about 1| c.c. Two-thirds actual size. 



waters of lakes, ponds, etc. In large deep lakes and in the open 

 sea the abundaDce is not likely to be so great, and rather larger 

 quantities of water will be necessary. 



In the centrifuging process itself there are two important 

 questions to be considered, namely, the speed of the centrifuge 

 and the length of time it must be kept running. In a general 

 way it is probably true that a low speed for a long time will be 

 as effective in concentrating the organisms at the far end of the 

 tubes as a high speed for a shorter time. In practice, however, 

 I believe, from my own experience, that there are some organisms 

 which will remain suspended in the water for an almost indefinite 

 time if the centrifugal force does not reach a certain amount, 

 and in order to concentrate these it is absolutely necessary to 

 run the centrifuge at a great speed. Now, for those who, 



