174 Prof. Tyndall on the Absorption and 



pumice-stone moistened with sulphuric acid, no such eft'cct was 

 observed. The needle moved steadily in one direction until its 

 maximum deflection was attained, and this deflection showed that 

 in all cases radiant heat was absorbed by the air within the tube. 

 These experiments were commenced in the spring of 1859, 

 and continued without intermission for seven weeks. The course 

 of the inquiry during this whole period was an incessant struggle 

 with experimental difficulties. Approximate results were easily 

 obtainable; but I aimed at exact measurements, which could not 

 be made with a varying source of heat like the copper ball. I 

 resorted to copper cubes containing fusible metal, or oil, raised 

 to a high temperature; but was not satisfied with their action. I 

 finally had a lamp constructed which poured a sheet of gas-flame 

 along a plate of copper ; and to keep the flame constant, a gas 

 regulator specially constructed for me by Mr. Hulet was made 

 use of. It was also arranged that the radiating plate should 

 form one of the walls of a chamber which could be connected 

 with the air-pump and exhausted, so that the heat emitted by 

 the copper plate might cross a vacuum before entering the expe- 

 rimental tube. With this apparatus I determined approximately 

 the absorption of nine gases and twenty vapours during the 

 summer of 1859. The results would furnish materials for a 

 long memoir; but increased experience and improved methods 

 have enabled me to substitute for them others of greater value; 

 I shall therefore pass over the work of these seven weeks with- 

 out further allusion to it. 



On the 9th of September of the present year (1860) I resumed 

 the inquiry. For three weeks I worked with the plate of copper 

 as my source of heat, but finally rejected it on the score of in- 

 sufficient constancy. I again resorted to the cube of hot oil, 

 and continued to work with it up to Monday the 29th of 

 October. During the seven weeks just referred to, I experi- 

 mented from eight to ten hours daily; but these experiments, 

 though more accurate, must unhappily share the fate of the 

 former ones. In fact the period was one of discipline — a con- 

 tinued struggle against the difficulties of the subject and the 

 defects of the locality in which the inquiry was conducted. 



My reason for making use of the high sources of heat above 

 referred to was, that the absorptive power of some of the gases 

 which I had examined was so small that, to make it clearly 

 evident, a high temperature was essential. For other gases, 

 and for all the vapours that had come under my notice, a source 

 of lower temperature would have been not only sufficient, but 

 far preferable. I was finally induced to resort to boiling water, 

 which, though it gave greatly diminished effects, was capable of 

 being preserved at so constant a temperature that deflections 



