Sjoeedy Volumetric Determination of Carbonic Acid, 183 



from its adhesion to the sides of the bell- jars on their being raised, I 

 thought there would be a difficulty in obtaining a fixed volume of air 

 within them. However, glycerine was tried, and the objection found 

 to be less than I had imagined, but glycerine absorbed water from 

 the air, and in due time would give it out to the dry air submitted 

 to analysis, so that glycerine could not be used without a protection 

 from atmospheric moisture. It then occurred to me that some kind 

 of oil might be made to float on the glycerine, thus completely 

 isolating it from the external air, and almond oil — which does not 

 thicken by exposure to the atmosphere — was adopted. The bath of 

 glycerine covered with almond oil in which the bell- jars are now 

 immersed was prepared in October last ; there is about one hundred- 

 weight of glycerine in each tank, and I tested the glycerine of one of 

 the tanks for water four months and a half after it had been, it may be 

 said, in daily use. This was effected by placing a weighed quantity of 

 the glycerine for about half an hour under a bell- jar over sulphuric 

 acid, and after that time it was found to have lost absolutely no weight. 

 The layer of almond oil had, therefore, perfectly answered its object. 

 The oil for confining the air to be analysed, ensured the absence of 

 any vapour-tension, or any possible loss of gas from absorption, and, 

 moreover, by lubricating the bell-jars prevented the glycerine from 

 adhering to them, the oil itself, from its lightness, running rapidly 

 down the sides of the receivers on their being raised or depressed. 

 A simple experiment demonstrates the action of the oil. Let a glass 

 tube opened at both ends be closed at one extremity with india-rubber 

 tubing and a pinch-cock, and then glycerine be poured into the tube. 

 By pressing on the tubing the glycerine is driven up, and on re- 

 leasing the pressure it slowly subsides adhering to the glass ; but if 

 a little almond oil be poured over the glycerine, on compressing the 

 tube the glycerine and oil are raised, but on releasing the pressure 

 the glycerine runs down quickly to its former position without 

 adhering to the glass. 



The bell-jars or air-holders gave me no little trouble. They were 

 made at first of thin sheet iron, but this, after much labour, was found 

 inadequate, as the changes of temperature of the external air affected 

 the gas through the metal, too rapidly to allow of a sufficiently fine 

 correction. Every possible contrivance was adopted to overcome this 

 difficulty. The tanks and bell- jars were enclosed in a wooden case with 

 glazed doors opening sideways, and tinfoil shields were placed round 

 the bell-jars so as to shelter them from any heat radiated from the out- 

 side, but these arrangements were too complicated and the shields had 

 to be given up. I came to the conclusion that the only plan would be 

 to keep the bell-jars under water during the whole of the analysis, and 

 this was done by making use of jacketed bell -jars, the space inside the 

 jackets being filled with water. A completely new instrument was then 



