Constant- Volume Air-Tliermometer. 155 



filled -with such a quantity of air that the pressure is approxi- 

 mately that of a normal atmosphere when the temperature is 

 freezing. For, if the quantity of air be considerably greater 

 than corresponds with this condition, there is a loss of range 

 in the instrument; whereas, if there be but a small quantity 

 of air, there is a tendency for the liquid of the yolume-gauge 

 to be drawn oyer into the bulb when the temperature of the 

 room comes down (as in winter it may) to about the freezing- 

 point, unless the instrument be left with the three-way stop- 

 cock closed and the air under diminished pressure. For 

 special circumstances the quantity of air may be made to suit 

 the conditions; for, as Regnault has shown, the results 

 obtained with the instrument are but yery slightly affected by 

 the initial pressure of the air, and this with very wide limits; 

 and by commencing at common temperatures w T ith air of 

 small density, yery low pressure, the upper limit of the range 

 may be extended without increasing the length of the inano- 

 metric tubes. 



The filling I accomplish in the following way: — The proper 

 quantity of liquid is first introduced into the yolume-gauge, 

 and the stop- cock t helps in introducing the liquid and in 

 adjusting the quantity. For this and the subsequent opera- 

 tions I use a good Bunsen water-aspirator, with a Woulfe's 

 two-necked bottle interposed between the aspirator and the 

 work, and a good length of small-bore non-collapsible india- 

 rubber tubing. With the india-rubber tubing the apparatus 

 to be exhausted can be turned into any required position 

 while the exhaustion is being carried on, and air-bubbles can 

 be got rid of with ease. 



When the yolume-gauge has been supplied with liquid, I 

 connect the three-way stop-cock t' to the aspirator, and draw 

 the whole of the liquid up into the bulb V and the tube lead- 

 ing up to the stop-cock itself. The size of the bulbs and of 

 the tubes is, as has been explained, such that when this has 

 been done the bulb b is empty as well as the tubes on the left- 

 hand side of the gauge almost down to the bend. The three- 

 wav stop-cock is then closed, and the aspirator disconnected. 



I now, with the help of a temporary three-way stop-cock, 

 connect together the tail-piece of the bulb shown in fig. 3, 

 the aspirator, and a train of drying and purifying tubes 

 (sulphuric acid and caustic potash). The arrangement is 

 such that, on turning the tap of the three-way stop-cock into 

 position No. 1, the aspirator draws the air out of the bulb; 

 while, on turning it into position No. 2, air flows into the 

 bulb passing through the drying tubes. The bulb is emptied 

 and refilled many times; and during the process the bulb and 



