434 
MR. J. T. BOTTOMLEY ON THERMAL 
the McLeod gauge as modified by Mr. C. H. Gimingham, # and shown in figs. 3a and 3b. 
The modification consists in supplying at the top of the volume-tube, as commonly 
constructed, a short piece (3 centims.) of fine thermometer tubing carefully calibrated. 
For the lower exhaustions the air is compressed into the wider portion of the volume- 
tube ; but for the higher exhaustions it is compressed into the divisions of the thermo¬ 
meter tube. To avoid errors from capillarity, there are two pressure tubes: that shown 
on the left in the diagram being a portion of the same tube as the wide part of the 
volume-tube, while the tube on the right in the diagram is made of a piece of the 
same thermometer-tube as is used for the finer part of the volume-tube. The mercury 
rises, of course, simultaneously in both pressure-tubes. It is usual, I think, to work 
the McLeod gauge by means of mercury from the main reservoir; but it will be found 
far more convenient to have an independent mercury reservoir as shown in the 
diagram, and, indeed, as it was originally described by the inventor. 
In my experiments up to the present time I have not introduced the iodine- 
sulphur-silver tube proposed and used by Crookes! as a stopper against the mercury 
vapour of the pump ; and, as the McLeod gauge measures only the pressure of that 
which is not collapsible in the vacuum space, its results are rendered imperfect for my 
purposes by the existence of mercury vapour. I must confess, however, that I 
cannot feel confidence in the power of finely divided silver or copper for stopping 
the vapour of sulphur out of the vacuum space, and unfortunately the testing 
spectroscopically, or otherwise, for vapour of sulphur is not so easy as for vapour of 
mercury. In experiments on radiation of heat the presence of any vapour whatever 
vitiates the results just to the extent to which it is present. In this connection I 
must also call attention to the difficulty of avoiding vapour from the material used in 
the drying tubes of the pump. It is now usual to employ drying tubes of phosphorus 
pentoxide; but it is extremely difficult to prepare this material perfectly free from 
small unburnt particles of phosphorus, and these will undoubtedly give rise to vapour 
of phosphorus. It may, I think, be doubted whether, after all, sulphuric acid is not 
in this respect a better drying material than phosphoric anhydride. I propose, 
however, shortly to make a series of experiments in which all vapours shall as far 
as possible be removed from the vacuum space, and afterwards stopped out of it by 
the interposition of a freezing chamber. 
Another reason, however, prevented my using Mr. Crookes’ mercury stopper, and, 
as it has also thrown great difficulty in the way of my measurements of the vacuum, I 
desire to mention it here. In determining the radiation, as will be seen presently, 
it is necessary to record simultaneously the energy lost and the pressure. But, 
unfortunately, I have been unable to find a platinum wire which does not incessantly 
give off small quantities of gas at high temperatures. This would be of no consequence 
at moderate pressures (xoo mm., for example) : but in extreme vacuums the effect is very 
* ‘ Journal of the Society of Chemical Industry,’ 1884. 
f ‘Phil. Trans.,’ 1885, p. 693. 
