6 THOMSON, Presidential Address. 



"Evaporation and /Springs," " True Rower of Fluids to Conduct 

 Heat," " Experiments and Observations on Heat and Cold Pro- 

 duced by Mechanical Condensation and Rarefaction of Air," 

 " Experimental Essays on the Constitution of Mixed .'Gases," 

 and " Meteorological Observations made at Manchester." John 

 Dalton was born at Eaglesfield, in Cumberland, 5th September, 

 1766. His father was a weaver of woollens in his own cottage 

 He was taught at a schoo(L ! of the Society of Friends, to which 

 the family belonged'. Sfo quickly did he acquire knowledge that 

 he was appointed at the age ,of 12 ;to conduct the school at 

 which he had been as a scholar. At ,14 he went to Kendal as 

 assistant in the school of Ms cousin. His first attempts at writing 

 were sent to The Gentlemen's Magazine, and) he received prizes 

 for the best answers, to the mathematical and other questions 

 propounded in that periodical. .When in Kendal he made the 

 acquaintance of Mr. Gouch, an eminent scientist, although blind 

 from birth. Through his 'influence he .obtained in 1793 the posi- 

 tion of Teacher of Mathematics and Physics in the New College, 

 Manchester (which was a continuation of the Warrington 

 Academy), and afterwards be spent the greater portion of his 

 life in the service of this Society. As soon as his great abilities 

 were recognised he was appointed Secretary of the Socie'ty 

 with his laboratory in the present building. He afterwards was 

 appointed President, and for 'about forty years, till his death, he 

 was the sole manager qf the Society's affairs. 



In his examination of the mode of analysing air he dis- 

 covered that in using nitric-oxide to absorb oxygen it required 

 72 measures to absorb the oxygen from 100 measures of air; 

 and if he used more than that of nitric-oxide, or more of air, 

 he got an excess of one or the other. Thlis led him to the 

 consideration of definite quantities of elements or compounds 

 uniting with each othiejr, and hie 1 argued that if a pound weight 

 Of one material, combined with a pound weight of another, 

 that half a pound would combine with half a pound, and so 

 that this relative proportion would continue to the smallest 

 conceivable weights. This gave him the idea of the Atomic 

 Theory', .He then represented these by balls, assuming a 

 hydrogen ball to be /black' afrid an oxygen ball white; then 

 water would be represented by one black ball joined to one 

 white ball, and inoi half ball can be ( used and noi confusion of 

 fractions. This was found to suit all the known facts in 

 chemistry. If (the quantity of water weighs 9, the hydrogen 

 would weigh 1 and the oxygen 8. The Atomic Weight of 

 hydrogen was therefore taken as unity, and the oxygen as 8, 

 and the [balls always represented these relative weights, and 

 the chemical combinations always took place in these definite 

 weights for each element. Thus Iron was found to be 128, and 

 in combining with oxygen; it took up 8 parts or multiples of 8, 



