higher temperature and the concentration required (0.013 vol.- 

 raol.) is almost at the limit for ordinary temperatures. If the 

 larger crystals that are left when solution is nearly complete 

 are crushed with a glass rod the completion of the operation is 

 hastened. 



It is well to have a chemical determination (analysis for one 

 element) made of each single-salt stock solution (or at least of 

 the first lot of solution made from a given bottle of salt) so as to 

 determine whether the concentration called for is actually 

 attained. 



The Mixed Stock Solutions and the Nutrient Solutions Them- 

 selves. To prepare any mixed stock solution, find the three par- 

 tial molecular salt concentrations for that particular solution 

 (tables I-VI) and multiply each of these three values by the 

 factor given for this purpose. This factor is unity for solution 

 types V and VI ; it is 3.50 for types I and III, and 3.0 for types 

 II and IV. From the three values thus obtained (and from the 

 molecular concentration of each of the three corresponding 

 single-salt solutions) calculate the number of cubic centimeters 

 of each single-salt stock solution needed for making a liter of the 

 mixed stock solution in question (or several liters; for simplicity 

 these directions are made to read for a single liter). Add the 

 three numbers thus secured and subtract the sum from 1000, 

 to give the number of cubic centimeters of water required. Place 

 this amount of water in a suitable bottle and proceed to add the 

 proper amount of each ol the three single-salt solutions, begin- 

 ning with the least concentrated of these and ending with the 

 most concentrated, and shake thoroughly to insure mixing as 

 thiS liquid runs in from the burette. Of course other procedures 

 may be followed; this one is simple and requires no large volu- 

 metric flasks, but it does require the careful measuring of the 

 three single-salt solutions and of the water. Throughout all 

 making of solutions it will be well to estimate the range of error 

 introduced and to keep record of this for each step of the pro- 

 ceeding. (Thus, 1 cc. as actually used may be, perhaps, anything 

 between 0.98 and 1.02 cc, etc. This error will of course depend 

 upon the burettes used, the temperature variations, the care 

 exercised by the operator, etc.) 



It will be noted that the mixed stock solutions are more con- 

 centrated than the corresponding nutrient solutions themselves 

 (as prepared to have osmotic values of 1.00 atm.), excepting for 

 types V and VI. In these two cases the mixed stock solutions are 

 the 1.00-atm. nutrient solutions themselves. 



37 



