260 Prof. Tyndall on Recent Researches on Radiant Heat. 



But I have not relied on the inspection of the outer surface 

 alone of my rock-salt plates. I have taken my apparatus 

 asunder fifty times and more, on occasions when I had most 

 reason to expect precipitation, but have not been able to find a 

 trace of moisture on my plates. 



This, however, did not entirely satisfy me, and I therefore 

 made an arrangement of the following kind : — An India-rubber 

 bag was filled with air and subjected to gentle pressure. By 

 a suitable arrangement of cocks and T-pieces, this air could be 

 forced either through a succession of tubes containing fragments 

 of marble moistened with caustic potash and fragments of glass 

 moistened with sulphuric acid ; or through a similar series in 

 which fragments of glass were moistened with distilled water. 

 A current of either dry air or damp air could be thus obtained 

 at pleasure ; and my object then was to get either the dry air or 

 the wet air, under precisely the same conditions, into an open 

 tube. To effect this, matters were so arranged that either cur- 

 rent could be discharged into the same narrow glass tube. This 

 glass tube was left in undisturbed connexion with one end of my 

 experimental tube, while the other end was connected with the 

 air-pump. The plates of salt were entirely abandoned, the expe- 

 rimental tube was separated from the " front chamber " de- 

 scribed in my memoir, and a distance of a foot intervened be- 

 tween the radiating surface and the adjacent open end of the 

 tube. In front of the other open end of the experimental 

 tube was my thermo-electric pile, the " compensating cube" 

 being applied in the usual way. By pressing the bag and gently 

 working the pump, I could, to a great extent, displace dry air 

 by moist, and moist air by dry. And in this way, without any 

 plates of rock-salt whatever, I verified all the results that I had 

 obtained with them. I have executed similar experiments in the 

 case of all other vapours that I have examined, and find that 

 with them, as well as with aqueous vapour, my plates of rock- 

 salt are perfectly to be relied on. 



Whence, then, the difference between Prof. Magnus and myself? 

 I am quite persuaded that no greater care could be bestowed 

 upon scientific work than Prof. Magnus bestows upon his ; and it 

 is the perfectly accurate nature of his experiments which renders 

 the explanation of the differences between us an easy task. 



Let me, however, first ask attention to what I may call a case 

 of internal evidence. I think the mere inspection of the drawing 

 of my apparatus in the ' Philosophical Transactions ' will show 

 that there was a good deal of thought and labour expended in 

 the construction of it. To one part of it especially I would di- 

 rect attention. In front of the experimental tube is a chamber 

 which is always kept exhausted, the radiant heat thus passing 



