Reilly and Hickinbottom — Volatile Fatty Acids. 523 



distilling liquid was also used to boil the water in the copper jacket. When a 

 vapour containing components of different volatility is cooled, the less volatile 

 component tends to be removed the more quickly. For acetic acid and water, 

 the acid being the less volatile, it is removed more quickly than the water 

 from the vapour phase ; consequently distillation constants for acetic acid will 

 be less when cooling takes place. This is shown by results given in Table 3. 

 For comparison the results from an experiment using the type of apparatus 

 finally adopted are also given. 



To prevent any appreciable variation in the volume of the liquid in the 

 flask, the water (free from C0 2 ) is run from a reservoir through a 10 c.c. 

 burette, the jet of which leads through a rubber bung fitted with a small tap- 

 funnel. By this means the flow of the water can be regulated by two taps, 

 the level of the water in the tap-funnel being kept constant. This secures a 

 uniform flow, accurate adjustment being left to the burette tap, after the 

 other tap has been set. 



The importance and also the difficulty of collecting an exact amount of 

 distillate in each fraction have been emphasized several times. In all the 

 experiments in the present work approximately 10 c.c. were collected in 

 weighed 10 c.c. measuring cylinders, the exact amount of distillate being 

 obtained by weighing the full cylinder. As the concentration of the acid is 

 small, a density of 1 may be taken for the solution. Each cylinder, as filled, 

 was weighed, and the contents then poured into a small, numbered flask. 

 The cylinder was washed twice with small quantities of water (free from 

 C0 2 ), and the washings added to the same flask. The stoppered numbered 

 vessels (1 to 10) containing the various fractions were left aside and the 

 titrations carried out after distillation was completed. Immediately before 

 collecting a new fraction the level of the water in the burette was restored to 

 zero, and the rate of flow so regulated that the volume run in from the 

 burette corresponded with the volume distilled, as measured by the amount 

 of liquid in the measuring cylinder. It was found that equal quantities of 

 distillate were collected in approximately equal intervals of time. 



The acids used in the determinations of distillation constants were 

 carefully purified by repeated fractionation, using a five-section " Young 

 evaporator stillhead," when the quantities available permitted. Usually a 

 fraction boiling over a range of O^-O - ^ 0. was used. 



Formic Acid obtained from Paulene Freres, Paris, described as pure, and 

 containing 98 per cent, anhydrous formic acid was fractionated once, the 

 fraction boiling at 100 , 2-100 , 5°C. (uncorr.) being taken. The distillation 

 constants are given in Table 4. 



4m2 



