in vacuo and in Gases. 277 



variable proportions, taking care never to add sufficient sulphuric 

 acid to allow any portion ofthat substance to pass by distillation. 

 Into the neck of the balloon I fitted two wide copper tubes, 

 placed one within the other like the tube of a telescope. The 

 upper tube carried towards its summit two lateral tubulures, 

 affording an exit for the vapour ; its upper orifice was closed by 

 a cork, through which the stem of a very sensitive thermometer 

 was passed. By this arrangement it was easy to place the bulb 

 of the thermometer in any region of the balloon, still keeping 

 the entire mercurial column in the vapour. 



Operating in this manner, it is soon seen to be impossible 

 to find any position in the balloon, where the bulb of the ther- 

 mometer does not constantly get covered with water, which 

 falls back, drop by drop, into the boiling liquid. Now every one 

 will understand that if the instrument be constantly moistened 

 with condensed vapour, it can never indicate a higher tempera- 

 ture than that at which the pure liquid boils under the same 

 pressure. It is therefore evident that no experiment in which 

 the thermometer becomes wet can prove anything in favour of 

 Rudberg's law ; and there can be no doubt that this has taken 

 place in the experiments of that physicist. 



The greater part of the water which runs down the thermome- 

 ter is produced by condensation on the upper part of the stem. 

 In order to prevent this water from reaching the reservoir, I 

 attached to the stem immediately above the reservoir a very thin 

 metallic disc, which collected it ; a second similar disc, attached 

 to the former by means of thin wires hung below the reservoir, 

 so as to protect this at once from the radiation of the super- 

 heated liquid and from the drops of the solution which are 

 always projected in abundance by boiling liquids. The bulb of 

 the thermometer was not more than 8 millimeters in diameter. 



Even with this arrangement it is very difficult to place the 

 thermometer in such a manner as that the bulb shall not be- 

 come wetted. As long as the bulb is at a distance of more than 

 3 or 4 centimeters from the boiling solution it always becomes 

 wet, and then it can never indicate any other temperature than 

 that of the ebullition of pure water. But when it is lowered so 

 as to go nearer the liquid, the temperature rises, the reservoir at 

 the same time becoming dry. The temperature thus continues 

 to rise until the bulb touches the liquid. 



The region of the balloon in which the thermometer indicates 

 a temperature above that of pure boiling water, may usually be 

 recognized even by sight ; it is that in which the inner walls of 

 the balloon remain dry, whilst the upper portions of its walls 

 are always moistened by condensed drops. The height of the 

 stratum of super-heated vapour depends upon the temperature 



