Measuring the Capacity of Gold-leaf Electroscopes. 245 



At the end of Table II. certain liquids definitely regarded 

 as associated are dealt with. The considerable excess of 



-— over — ^ . -~ found is clearly to be attributed to work 



done (heat absorbed) in splitting up the associated molecules 

 as they pass into the vapour phase. In this connexion the 

 agreement found with acetic acid is interesting, in view of 

 the fact that the vapour of the acid is known to be associated. 

 Seeing that </, the molecular diameter, varies but slowly 

 with increasing molecular weight, we may use the relation 

 to make a rough estimate of the " association factor/' ?i, for 

 liquids the vapours of which are completely dissociated, by 

 writing : 



p.n.M_ Lj 



where M is the gramme molecular weight of the simplest 

 or vapour form of the molecule. In this way the following 

 values are found for n : — 



Substance. T. n. -r, a , ■ u 



Kainsay-ohields. 



Water 15 17 



Methyl alcohol 23 23 



Ethyl alcohol 1*9 17 



Chlorine (as 01) --72 2-6 



The value of n calculated by the Ramsay-Shields method 

 is recorded for comparison. 



XXII. A Method of Measuring the Capacity of Gold-leaf 

 Electroscopes. By A. T. Mukerjee, M.A., Pa'tna College *. 



Abstract. 

 The arrangements for usin# a Dolezalek electrometer for 

 accurate work, devised by V. H. Jackson and the author (con- 

 tinuation of earlier work described in J. A. S. B. vol. x. No. 6, 

 1914) and shown in working order at the fifth session of the 

 Indian JScience Congress at Lahore, 1918, have made it possible 

 to develop a quick and very accurate method of measuring 

 extremely small capacities, such as those of gold-leaf electroscopes. 

 Using a standardised (Gerdien) sliding condenser, the absolute 

 value of the capacity of the quadrant system, including a specially 

 designed connecter, is first determined by the method of mixtures. 

 The gold-leaf system of the electroscope is then charged to a known 

 voltage, adjusted so that when the charge is shared with the 

 quadrant system the final potential, as measured by the electro- 

 meter, is not far from one volt. As the capacity of the quadrant 

 * Communicated by the Author. 



