400 



Prof. B. Moore and Mr. H. E. Eoaf. 



[Apr. 12, 



Precautions in Preparing, Preserving, in Unaltered Strength, and 

 Diluting by known Amounts, of the Solutions Used. 



In working with the solutions, it is indispensable that due precau- 

 tions be taken against loss by escape of chloroform into the air during 

 the various manipulations. 



If a portion, for example, of a stock solution of known strength be 

 pipetted off for use in the densimeter, and then the stock bottle be 

 merely stoppered, the air space over the portion of solution left in the 

 bottle rapidly becomes charged with chloroform vapour at the expense 

 of the stock solution, and a second sample taken later from the same 

 bottle will be found to be weak and give a wrong result. The way to 

 guard against this is to fill the bottle with mercury up to the neck 

 immediately after drawing off, and at once stopper up. Similar devices 

 were employed in all dilutions to complete the process out of contact 

 with air. 



The stock solutions were made by direct weighing, by dropping from 

 a fine pipette, into graduated glass-stoppered flasks of 25, 50, or 100 c.c. 

 capacity, according to the amount required, immediately filling up to 

 the mark with the particular solvent, and setting at once upon a slowly 

 rotating disc, driven at such a rate that the chloroform globules have 

 just time to fall through the solution each time the flask is inverted. 

 In this way, a rapid solution is effected, so saving much time. Further, 

 in certain cases such shaking is indispensable, for a solution of 

 haemoglobin left in contact with even a small amount of chloroform 

 without continuous shaking soon forms a precipitated layer at the 

 bottom. 



The stock solutions were kept carefully stoppered, unless at the 

 moment of introducing the pipette to remove a portion for experiment, 

 and then the space within the bottle was always filled to the stopper 

 with mercury in the manner above described. 



We found much difficulty owing to leakage of chloroform during 

 dilution in the case of the strengths intermediate between the stock 

 strengths, until it occurred to us to make these dilutions in the 

 densimeter itself. For this purpose we always placed the mercury 

 level at 10 c.c. below the stopper, on the side of the densimeter in 

 which the mixing was to be carried out, then the proper amounts of 

 the two solutions necessary to give the desired strength was drawn 

 up in two graduated pipettes and run into the densimeter tube, the 

 stopper was then inserted, including only a minute bubble of air, and 

 now by thorough agitation of the iron stud through the fluid by 

 means of the electro-magnet a thorough mixing of the two fluids was 

 attained, then by raising the mercury-holder and slightly easing the 

 stopper 5 c.c. were allowed to escape, the mercury level being placed 

 accurately at 5 c.c. A seal of mercury was finally dropped in above 

 the stopper, and so the dilution was effected. 



