ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 727 



B. Technique.* 

 (1) Collecting: Objects, including- Culture Processes. 



Nutrient Media of "Standard" Reaction, f —Dr. J. W. H. Eyre 

 contributes a valuable article on the standardisation of nutrient media 

 by exact titration methods. After a short historical review of the chief 

 methods adopted in the past for obtaining media of definite composition 

 and reaction, the author deals with the indicators, the reaction of the 

 raw materials, neutralising solutions, the optimum reaction, and the 

 preparation of standard media. The procedure for standardising media 

 is as follows : — 



Solutions required. — — NaOH, accurately standardised. — NaOH, 



accurately standardised ; 0*5 p.c. solution of phenolphthalein in 50 p.c. 

 alcohol (in bottle with pipette holding 0*5 ccm. through the cork). 



Apparatus required. — 25 ccm. burette graduated in tenths of a ccm. 

 25 ccm. measure or pipette. Bohemian glass flask, fitted as a wash- 

 bottle, filled with distilled water and kept boiling on a tripod stand. 

 Several 60 ccm. Erlenmeyer flasks or conical beakers. Some squares 

 of white blotting-paper. 



Method. — The burette is filled with —NaOH; 25 ccm. of the fluid 



medium are measured out into one of the flasks or beakers, the measures 

 rinsed out with a small quantity of boiling distilled water from the wash- 

 bottle and added to the medium already in the flask, then half a cubic 

 centimetre of the phenolphthalein solution run in. To this colourless 



fluid — NaOH is added cautiously from the burette until the end-point 



is reached, as indicated by the development of a pinkish tinge. A 

 control, a second, or even a third may be titrated, but such is the sharp- 

 ness of the end-point, that after a little experience with this indicator 



there will not be a greater difference than 0*1 ccm. of the — NaOH 



between the several estimations, and as a matter of fact, it is almost 

 impossible to overshoot the end-point of even the first titration by more 

 than 0-2 ccm. of the decinormal solution. From these estimations, 



the amount of - NaOH requisite to neutralise the remainder of the 



medium can be easily calculated ; and from this figure is deduced the 

 amount that is necessary to add to the remainder of the medium, in 

 order that it may still remain acid to phenolphthalein to the extent of 

 1 p.c. ; in other words, have a reaction of -\- 10. 



The differences in technique between this method and that recom- 

 mended by the Americans are : — 



(1) The use of 25 ccm. of medium instead of 5 ccm. of medium 

 + 45 ccm. boiling distilled water. 



* This subdivision contains (1) Collecting Objects, including Culture Pro- 

 cesses ; (2) Preparing Objects ; (3) Cutting, including Imbedding and Microtomes ; 

 (4) Staining and Injecting; (5) Mounting, including slides, preservative fluids, &c. 

 (6) Miscellaneous. t Brit. Med. Journ., 1900, ii. pp. 921-3. 



