ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 423 



chemistry and metallography, because these have been so adequately 

 treated in Dr. E. M. Chamot's recent " Micro-Chemistry." About one- 

 fourth of Page's book (pp. 312-44;-)) is taken up with materials of micro- 

 scopy and preparation of objects. Nearly three-fourths are given to the 

 elucidation and application of microscopic apparatus, and the remainder 

 is concerned with a very interesting historical sketch of lenses and 

 microscopes. There is also a good bibliography. 



Direct Microscopicai Counting of Bacteria in Water.* — B. E. 

 Nelson says that the estimation of the number of bacteria in water by 

 direct counting has several advantages over plating methods. Although 

 requiring more skill than the latter it occupies much less time and avoids 

 the necessity for preparing and sterilizing culture media. Plating 

 methods give low results in general, because there are always some 

 organisms present which fail to produce colonies, and clusters of 

 organisms cannot be distinguished from isolated individuals, since they 

 only give rise to single colonies. The results obtained by direct 

 counting are, on the other hand, liable to be high, owing to the inclusion 

 of dead cells in the counts. Direct counting gives opportunity for 

 observation of the various types of organisms present, altuough at the 

 sacrifice of the characteristic colony formations and other biochemical 

 reactions during growth on plates which are often used as diagnostic 

 points. 



In applying the method of direct counting to potable waters, which 

 are relatively clean, it is necessary to concentrate the organisms into a 

 small volume, e.g. those from a litre into 1-3 c.cm. In order to effect 

 such a concentration, whether by sedimentation, filtration, or centri- 

 fuging, it is necessary to use colloidal " flocking " materials, and the 

 most suitable appears to be zinc hydroxide, as after concentration this 

 can be redissolved by addition of ammonium chloride so as not to 

 interfere with the counting. The precipitate is formed by adding to a 

 litre of the water 1-5 c.c. of 5 p.c. zinc sulphate solution, followed after 

 two to three minutes by the proper amount of caustic soda (0"7-3'7 c.cm. 

 of N. caustic soda) ; the liquid is shaken thoroughly and allowed to 

 stand for a few minutes with occasional shaking. The organisms in 

 the water become entangled in this precipitate, and can be concen- 

 trated with it into a. small volume. The author studied three methods of 

 concentrating — viz. by allowing the precipitate to subside spontaneously, 

 by filtering it on a layer of sand, and by centrifuging ; in all cases 

 the precipitate, after separation from the bulk of the water, was finally 

 concentrated to 1 or 3 c.cm. in a small medical centrifuge. Full working 

 details of the methods adopted are given. It is estimated that the 

 precipitate separated by filtration on sand contained more than 99 p.c. 

 of the bacteria originally present in the water ; that separated by 

 centrifuging contained about 92 p.c, whilst that deposited spontane- 

 ously only contained 80-86 p.c. Whilst therefore the filtration method 

 proved most efficient, the centrifugal method was by far the most 



* Journ. Amer. Chem. Soc. No. 39 (1917) pp. 515-23, through Journ. Inst. 

 Brewing, xxiii. (1917) pp. 259-61. 



