1880.] Diurnal Variation of Carbon Dioxide in the Air. 343 



which our experiments point. Provided we admit that materials of the 

 comet contain ready formed hydrocarbons, and that oxidation may 

 take place, then the acetylene spectrum might be produced at com- 

 paratively low temperatures without any trace of the cyanogen spec- 

 trum or of metallic lines. If, on the other hand, we assume only the 

 presence of uncombined carbon and hydrogen, we know that the 

 acetylene spectrum can only be produced at a very high temperature, 

 and if nitrogen were also present that we should have the cyanogen 

 spectrum as well. Either, then, the first supposition is the true one, 

 not disproving the presence of nitrogen, or else the atmosphere which 

 the comet meets is hydrogen only, and contains no nitrogen." 



II. " On the Diurnal Variation in the Amount of Carbon 

 Dioxide in the Air." By George Frederick Armstrong, 

 M.A., F.G-.S., C.E., Professor of Engineering in the York- 

 shire College, Leeds. Communicated by Professor Thorpe. 

 F.R.S. Received April 12, 1880. 



Although a large share of attention has been given to the elucidation 

 of the causes which influence the amount of carbonic acid present in 

 the atmosphere during the day, no systematic observations with 

 reference to the relative quantities present in the air of the land during 

 the day and the night appear to have been undertaken since the well- 

 known experiments of the younger De Saussure at Chambeisy,* 

 upwards of 50 years ago (182b"-30), and a similar set by Boussingault 

 at Paris, f a few years later, until M. TruchotJ took up the question in 

 1873. But the results thus obtained cannot be said to be altogether 

 satisfactory. 



On the other hand, the question as to the existence of a diurnal 

 variation in the amount of carbonic acid in sea-air may be said to have 

 been fully worked out and set at rest by the careful and extensive set 

 of observations made some years ago by Dr. Thorpe§ on the air of the 

 Irish Sea and Atlantic Ocean. His results went to show that no 

 appreciable diurnal difference exists. 



As regards a diurnal variation in the case of land-air, the results of 

 De Saussure are somewhat contradictory; and latterly, what he him- 

 self considered as " one of the most remarkable results " in the whole 

 range of his inquiries — the discovery of a distinct diurnal variation in 

 the amount of carbonic acid — has been regarded as but very imper- 

 fectly established. 



De Saussure published his first set of results which were deduced 



* " Ann. de Chim. et de Phys.," vols, xxxviii (1828) and xliv (1830). 



f "Ann. de Chim. et de Phys.," [3], 10 (1844). 



% " Compt. Rend.," 77, 675. 



§ " Journ. Chem. Soc.," [12], v. 189. 



