166 
but in order to ascertain that the tube was cleared of air it 
was moved gently up and down to see if the mercury 
clicked, as it usually does when the tube is free from air, 
but the mercury did not move in the tube. The rapidity 
of the oscillation was thereupon increased until it became a 
violent shake, and as the mercury still remained firm it was 
clear that some very powerful force was holding it in its 
place. The tube, being in a vertical position, was then left 
in order that the barometer might be consulted. This was 
standing at 29 inches. After a few seconds, when the 
gauge was again examined, the column no longer reached 
the end of the tube, but stood at 29 inches. As it was sin- 
gular that the mercury should have quietly settled down 
after having resisted such violent shaking, the tube was 
again inclined until the mercury and acid came apparently 
up to the end of the tube ; but this time on the erection of 
the tube the mercury at once settled down. That is to say, 
it settled down gradually as the tube was erected. At first 
what appeared to be a very small bubble opened in the sul- 
phuric acid, and this enlarged as the top of the tube was 
raised. On again inclining the tube until it was horizontal, 
and examining it closely, a minute bubble could be seen in the 
acid, and it was this bubble which expanded as the tube was 
erected, and so allowed the mercury to descend. To get 
rid of this bubble the tube was turned down so as to allow 
the bubble to pass along the tube, but owing to its small 
size it would not pass many inches along the tube before it 
became fixed between the mercury and the glass. When 
the bubble came to a standstill at about 6 inches from the 
end of the tube the gauge was again erected, the bubble 
immediately began to move back, but so slowly that it was 
some seconds before it entered the region of no pressure. 
During this interval the mercury remained up to the end of 
the tube, but the bubble, as soon as it neared the top of the 
tube, expanded and rapidly rose to the top of the tube, 
