424 Influence of the Adhesion of Vapour on the Absorption of Heat . 



he has not succeeded in obtaining certain results. He con- 

 cludes with the complimentary remark that it might perhaps in 

 my hands yield better results. I am convinced that what is effi- 

 cient in my hands would be equally so in his. But he also 

 would have obtained satisfactory results by that method if his 

 apparatus had not been different from mine in exactly the most 

 essential part. If, in order to avoid the use of rock-salt or 

 other plates to close the tube, one end be used as a source of 

 heat for the pile as I arranged the apparatus, it is clear that 

 currents of air can only be avoided by holding the tube vertical 

 and heating it at the top. But if with such a position of the 

 tube it be constructed of metal, the part of the inside nearest 

 the end soon becomes heated, and sends its rays towards the pile. 

 The air at the same time in contact with this surface becomes 

 heated, and currents are formed which affect the temperature of 

 the pile and render an observation impossible. To avoid such 

 currents I used glass tubes, and the upper part to be heated 

 was blown as thin as it is possible for glass to be. The use of 

 metal for the boiling-vessel was also avoided ; and for this pur- 

 pose a thin glass vessel was so fused on the end of the tube to 

 be heated that its bottom closed the tube. The section of the 

 bottom, moreover, was only one-fourth that of the tube, so that 

 the heating might be limited to as small a surface as possible. 



I should have thought that this arrangement was suffici- 

 ently clear from the drawing which I had given of the ap- 

 paratus. In addition to this, there was a delay after each 

 pumping and admission of air until the tube and the pile had 

 completely acquired the temperature of the atmosphere, for 

 which more than a quarter of an hour was needed; during this 

 time the glass near the boiling-vessel completely acquired the 

 temperature of the neighbourhood. When the boiling water was 

 again poured in, and kept boiling by steam, after a very short 

 time, within which the glass in the neighbourhood could only 

 become very slightly heated, the observation could be made by 

 opening the screen in the interior of the tube. I am convinced 

 that with an apparatus similarly arranged Professor Wild would 

 have obtained the same results as I have. Although I admit 

 that the use of this apparatus, owing to its fragility and 

 the lengthened delay between one observation and another, is 

 neither easy nor pleasant, and although the certainty which 

 it admits leaves much to be desired, it has so far succeeded 

 that it has enabled me to recognize with certainty the deport- 

 ment of aqueous vapour. The diaphragms in the interior have 

 probably played the most important part in this ; for they only 

 allow a few reflected rays to reach the pile. 



The origin of the difference between Professor Tyndall and 



