RADIATION IN ABSOLUTE MEASURE. 
433 
used, does act perfectly in keeping as good a vacuum, I believe, as can be made, and 
which does not give off at common tenqieratures any vapour which can be detected 
by the tests known at present. This invaluable material is “ Siegelwachs,” # a small 
piece of which was given to me by Professor Quincke, with the remark that it would 
“hold a vacuum”; and with it I construct my vacuum-tube with a wire stretched in 
it in the following way. ab (figs. 2a and 2b) is a copper tube one metre long. The 
bottom is closed with a metal plate, which is soldered on : but in the centre of the 
plate there is a small hole through which is brought out the extremity of the hard- 
drawn copper spiral shown in the diagram. At the top of the copper tube there is 
soldered in a short piece of re-entrant tube of a nearly conical shape ; e is a glass 
tube which is prepared to fit the mouth of the tube ab. To do this, the glass tube, 
closed for the time at o, is heated to softening round about, and is blown into 
the tube aa as into a mould. It is then quickly withdrawn and carefully annealed. 
Lastly, two or three stout platinum wires are sealed into the glass piece at cl, and 
these, after the glass splint at o has been cut off, are brought out to a point to which 
one end of the radiation wire is silver-soldered. The other extremity of the radiation 
wire is attached to the spiral spring already inside the copper tube, but which for the 
purpose of silver-soldering is brought to the top a of that tube. When the soldering 
has been effected the tail-piece of the spiral is pulled down and out through the hole 
at b, and when all other arrangements are completed is soldered in its place. Pre¬ 
viously to this, however, the glass piece e is put into its place with an extremely 
minute quantity of “siegelwachs”; and then, with the help of a pointed glass rod, heated 
in a spirit lamp, the cement is put in drops round the junction of copper and glass, 
and carefully worked in so as to fill up every re-entrant corner, and make a coating 
everywhere convex outward except just where it joins the vertical part of the glass 
tube. Here the wax is thinned away very gradually, the greatest care being taken 
to make sure that all round the edges the cement is adhering perfectly to the glass. 
With a joint made as described I have maintained a high vacuum for weeks together; 
and, in fact, 1 have no reason for thinking that such a joint is not quite as good at 
atmospheric temperature as the usual joint made by fusion of glass on to the exhausting 
tube of the Sprengel pump. 
It is unnecessary for me to enter into details as to the production of high vacuums 
by means of the Sprengel pump. In this matter the splendid researches of Mr. 
Crookes have, in every detail, pointed the way for obtaining the best results; and 
the five-fall Sprengel pump of Mr. GiminghamI is admirably convenient for 
exhausting vessels of moderate capacity. In measuring the vacuums produced, I use 
* “ Siegelwachs ” is not similar to English sealing-wax (“ Siegellac”). It is a soft wax easy to mani¬ 
pulate and not liable to crack, and it shows no tendency to contract and draw away from the glass or 
metal to which it has been made to adhere. I obtain it from the firm of J. Gautsch, Konigliche Hof- 
Wachswaaren-Fabrik, Munich, whose name was given me by Professor Quincke. 
t ‘ Roy. Soc. Proc.,’ vol. 25, 1877. 
MDCCCLXXXVII.—A. 3 K 
