Co?istant- Volume Air-Thermometer . 



153 



capillarity of the tube d. Either mercury or sulphuric acid 

 may be used in the volume-gauge. I prefer sulphuric acid on 



Fiff. 3. 



account of its smaller density. The greatness of the density 

 of mercury, and the uncertainty of its capillary action, make 

 its use very liable to produce serious errors in reading. But, 

 on the other hand, in the case of sulphuric acid, the wetting 

 of the tubes, which constitutes its advantageous quality so far 

 as capillarity is concerned and gives regularity of capillary 

 action which mercury never possesses, renders watchfulness 

 nece.-sary to keep the acid well clear of the fine tube c. If 

 once the acid is allowed to enter that tube, it tends to form 

 beads and to make its way along it towards the bulb. 



The manometric tube is simply a U-tube capable of giving 

 a difference of levels of from 100 to 150 centimetres of mer- 

 cury, and wide enough to make capillarity very small and 

 difference of capillarities in the two tubes negligible. With 

 a tube giving a difference of levels of 150 centimetres, a 

 temperature of about 550° 0. may be reached, starting with 

 air at normal density at common temperatures. The difference 

 of levels may be read by means of a kathetometer, or, what is 

 preferable, the tubes themselves may be graduated to milli- 

 metres. The tubes which I use are graduated from a zero 



