DEAS ON SOLUBILITY. 797 
described by Géckel** for determining solubility in boiling liquids. 
This involved forcing the boiling solution by pressure through a filter 
into a weighed flask fitted by means of an adapter to a reflux con- 
denser: the solution so collected was weighed and analysed. The 
filter through which the hot solution passes was made in the form 
of a condenser kept hot by a jacket of water a few degrees above the 
boiling-point of the solution. 
1898 When studying the rate at which substances dissolve, 
’ Noyes and Whitney *? ** used the solute in the form of sticks 
varying in superficial area, and caused these to be rotated in small 
flasks. 
A very efficacious method of preparing saturated solutions is 
described by Koehler and Martini,*** involving the agitation of solute 
and solvent together in a tube fitted with a glass stirrer made in the 
form of an Archimedes screw; a form of stirrer now very generally 
used. 
1899 Pawlewski**> prepared saturated solutions at temperatures 
‘ ranging from 0° to 100° C. by agitating solvent and solute 
together in a tube, the agitation being effected by drawing a current 
of air through the liquid. The tube and also a weighed receiving 
flask were immersed in water at the required temperature, and were 
so connected together that a sample of the saturated solution could 
be transferred to the flask by suction whilst still immersed in the 
water-bath. 
1900 In the following year Hopkins *? suggested a more novel 
‘ method of preparing saturated solutions. Placing solvent and 
solute together in a tall cylinder, by a simple device he pumped the more 
concentrated liquor continuously up from the bottom of the cylinder and 
projected it on to the weaker solution above. 
1901 Immerwahr '* studied the solubility of precipitates containing 
’ heavy metals by measuring the potential differences between 
electrodes of those metals—such as mercury, copper, lead, cadmium, 
and zinc—and the saturated solutions of their less soluble salts: the 
saturated solutions being obtained by precipitating salts of the metals 
with excess of the precipitant. The concentration of the ‘ ions’ of the 
heavy metal in solution was calculated from the potential difference, and 
since the same excess of precipitant was used in all cases, the potential 
differences were regarded as being proportional to the solubilities of 
the salts. 
1902 The work of Noyes and Kohr 4? casts doubt on the validity of 
’ the electrical conductivity method as a means of determining the 
solubility of substances. These authors base their contention on their 
own estimation of the solubility of silver chloride in aqueous solutions of 
potassium chloride and hydroxide. The results which they arrived at by 
direct analysis led to the conclusion that silver hydroxide is dissociated 
only to the extent of 70 per cent. ; whereas Kohlrausch, when determin- 
ing the solubility of silver chloride by the conductivity method, assumed 
it to be completely dissociated. The solubility of sparingly soluble 
1908 salts was also studied by Abegg and Coz, '*’ who determined the 
‘ solubility of certain silver salts by measuring the potential differ- 
