Page:Proceedings of the Royal Society of London Vol 69.djvu/377

Rh manently on one scale of the balance during all the weighings, while the A flask was weighed, either exhausted or filled with oxygen (or nitrogen), under various circumstances according as the experiments required.

As the flask cooled in the liquid oxygen or air had to stand an internal pressure of from three to four atmospheres, it was considered expedient to select a spherical vessel of about 300 c.c. capacity, to which was sealed a narrow tube having a very carefully ground stop- cock at its end. Preliminary experiments were made to determine the change of volume of the flask when subjected to internal pressure. The flask, filled with air at five atmospheres pressure, was left for 24 hours with the stopcock closed, without showing any leakage. To determine the eftect of pressure on the capacity of the flask, it was filled with water under one atmosphere pressure, and again with water under three to four atmospheres pressure (which included the range of the observations) and the weights noted. From these the coefficient of expansion of the flask was found to be 0*000306 per atmosphere excess of internal over external pressure. The temperature coefficient of expansion of glass (cubical) was taken to be - 000025, which for a variation of temperature of some 200 altered the capacity of the flask by about 1*5 c.c. The content of the flask up to a fixed mark on the neck below the stopcock was determined to be 315*973 c.c. at 17 C. ; the content between the mark and the stopcock was determined both by measurement and by the weight of mercury it contained, and was found to be 0*127 c.c.

Before each experiment the A flask filled with the gas under observa- tion was exhausted to a pressure of from 2 to 4 mm. of mercury (which was afterwards involved as a correction in reducing the observations) and weighed, the weight a which had to be added to its scale pan to balance the B flask being noted. The A flask was then filled with carefully purified oxygen (or nitrogen), and the temperature of the flask and contents (still in communication with the gas reservoir) was lowered by immersing it up to the mark in liquid oxygen (or air) until the gas ceased passing into the flask, and the pressure was finally equalised to that of the atmosphere. This is really the most important part of the manipulation, as the accuracy mainly depends on giving sufficient time to the cooling, while at the same time taking care to avoid any excess of pressure that would necessarily lead to liquid condensation on the glass surface. During the rapid inrush of gas, it is advisable to keep the pressure well below that of the barometer at the time, and to finally adjust the pressure at the end of the absorption. The vessel containing the liquid oxygen had a cardboard cover which crossed the neck of the flask at the mark, thereby preventing the cooled vapour of the oxygen from freezing the stopcock. The remain- ing portion of the tube between the mark and the stopcock, namely,