OSMOTIC PRESSURES DERIVED FROM VAPOUR-PRESSURE MEASUREMENTS. 323 
Even now water was occasionally deposited where it had no business to be. The 
cause of this was finally traced to the method we had hitherto employed in cleaning 
the vessels and their joining tubes. We had made a point of always cleaning out 
everything with “chromic” acid, and washing this acid out with distilled water (six 
washings in all) and then drying the parts in a current of air. It was noticed that 
dew formed at the places where the last drops of water evaporated off. This at once 
suggested that on evaporation the water left behind it some soluble substance, which, 
during the subsequent passage of saturated air, formed a solution having a smaller 
vapour pressure than pure water, hence the deposits. That the water actually did 
contain soluble matter was easily shown by evaporating off 100 c.c. in a platinum 
dish. 
Efforts to obtain water free from this soluble matter failed completely, even 
though it was redistilled four times with all the usual precautions for obtaining 
“ conductivity ” water. That the deposit shown on the platinum dish did not come 
from the atmosphere was proved by evaporating off the samples in a desiccator over 
sulphuric acid. Eventually the vessels were cleaned by giving them a final washing 
with pure alcohol* and drying in a current of warm air. Although this procedure 
was successful in reducing the trouble, yet the platinum tube (bore 6'5 mm.) which 
joined vessels 4 and 5 (the water to the sulphuric acid) generally contained about 
CT0030 gr. of water. 
No valid explanation of this phenomenon has yet been found, t and it is the more 
inexplicable in that three experiments have lately been carried out in which both the 
third and fourth vessels were reserved for water, with the result that the quartz tube 
between 3 and 4 contained less than O'OOIO gr., while the platinum tube retained 
the usual quantity. One of these experiments was arranged so that hydrogen was 
passed through the train of vessels instead of air. 
We would draw particular attention to this question of the deposition of moisture, 
for there seems to be no doubt that it may have been a source of error in previous 
determinations of the absolute vapour density of water in air. 
The Experiments at 0° C.—These experiments were all made with Apparatus Aa. 
The cane sugar and the a-methyl together with the method of estimating the water 
content are described in ‘Hoy. Soc. Proc.’ A, vol. 92, 1916, p. 479. The following is 
an example experiment. 
* The alcohol used was Kahlbaum’s purest, re-distilled twice. Samples from the first distillation left 
deposits on the platinum dish, which were partially soluble in water, but the second distillation was found 
to be pure. 
t If the conclusion come to in the note added at the end of the paper be correct, namely, that the last 
branch of vessel 4 is at a higher temperature than the remainder of the bath, then condensation in the 
platinum tube is to be expected. 
