1 8 GEE, Dalton's Lectures and Lecture Illustrations. 



elastic fluids during the last 50 years. Indeed, before 

 that period, the atmosphere may be said to have been 

 considered as a homogeneous fluid. As soon as it was 

 discovered that elastic fluids of very different chemical 

 properties existed in the atmosphere, and were mostly or 

 always found in a state of intimate mixture, it became a 

 question how this mixture was effected and maintained ; 

 especially as the elastic fluids are of very different specific 

 gravities. A notion was adopted that it must be the 

 result of chemical affinity, and analogous to the solutions 

 of salt, sugar, &c, in water. But the analogy does not 

 hold ; for salts do not dissolve in water without agitation ; 

 whereas airs dissolve in each other without agitation. 

 Also, salts produce cold, or heat, and condensation of 

 volume ; whereas airs produce none of these. 



Considerations of this kind, together with the enor- 

 mous disproportion in which the elastic fluids constituting 

 the atmosphere are found mixed, put me upon thinking, 

 about 25 years ago, whether a more rational account 

 might not be given of the constitution of the atmosphere. 

 This led to such views of the subject as terminated in a 

 new theory of chemical combination in general, or what 

 is called the atomic theory. — But this is a subject we must 

 not enlarge upon at present. 



The Manchester lectures realised £27. 12s. od. The 

 same course was also given at the Birmingham Philo- 

 sophical Institute, for which he received 40 guineas. 



1827-29. 



During these years his lectures in Manchester were 

 devoted to Heat and Chemistry. The printed syllabuses 

 for the three years differ but little and are chiefly based on 

 his " New System of Chemical Philosophy." His account 

 books show that he received on an average about £Zo for 



