526 
Proceedings of the Royal Society of Edinburgh. [Sess. 
water. When constant values were reached, the temperature of the bath 
was raised and a new observation taken. 
Criticism of the Method. — It will be seen that the isoteniscope is free 
from the sources of error which affect the older forms of apparatus for 
determining vapour pressures. For example : — 
1. The isoteniscope substitutes for mercury a confining fluid of low 
specific gravity, and thus eliminates the error in levelling involved in some 
static methods. 
2. The apparatus dethrones mercury from its position as sole confining 
fluid, and substitutes a wide choice of liquids. In consequence, all errors 
due to the volatility of a foreign confining fluid, and to solubility of the 
substance in or interaction of the substance with such a confining fluid, 
disappear entirely. 
3. By being adapted to use in a liquid bath (with violent stirring), the 
apparatus reduces to a minimum all errors due to unsteadiness of the 
temperature, and to inequality in the temperatures of the substance, its 
vapour, and the thermometer. 
4. By providing a simple method for expelling air bubbles and adhering 
and dissolved gases, and for repeating the expulsion until the success of the 
operation is demonstrated, the isoteniscope removes a source of error (and 
of total uncertainty as to the amount of that error) which destroys all con- 
fidence in the exactness of the results obtained by the older static methods. 
5. By permitting the making of an extended series of observations with- 
out using more than half of the sample, the apparatus obviates an error due 
to concentration of involatile impurities in the residue which affects some 
dynamic methods 
6. By allowing the expulsion of accumulated gas immediately before a 
reading, the apparatus makes possible accurate measurements with many 
substances which decompose slowly to give a permanent gas. By the older 
static method, accurate measurement in such cases was necessarily 
impossible. 
Aside from this avoidance of certain sources of error, the apparatus 
possesses some noteworthy advantages : — 
1. A small amount of material suffices. 
2. The filling of the apparatus, and the manipulations involved in a 
measurement, are incomparably simpler than in any other static method. 
3. The correction for dilatation of the bulb of the mercury thermometer, 
which some dynamic methods involve, is avoided. 
4. The method permits measurements with all reasonably stable liquids, 
no matter how active, chemically, they may be. 
