on the Chemical Compounds of Azote and Oxygen, &c. 243 



But to return to the subject of atmospheric air and its gases. 

 — " Soon after this appeared (nieaniiii^ his hypothesis of mixed 

 gases) Berthollet, in his Researclies into the Laws of Chemical 

 Affinity, announced a new explanation on the phsenomena of 

 mixed gases. According to this eminent chemist, there are two 

 species of affinity ; the one strong, tlie other weak :— the stiong 

 affinity makes bodies unite chemically ; the weak only serves to 

 diffuse them througii each other without producing condensation 

 of volume ; its effects may be called solution or dissolution. Of 

 this kind, he conceives, is the mutual action of gases that do 

 not combine, ami that it operates just the same upon gases in- 

 clined to couibination or not: — thus a mixture of carbonic acid 

 gas and hydrogen is subject to this weak or slight affinity just as 

 ninch as one of oxygen and hydrogen. Something similar to 

 this is supported by Murray in his Elements of Chemistry, and 

 by Dr. Thomson in the third edition of his Chemistry. Mr. 

 Gough in different papers in the Manchester Memoirs, and in 

 some essays of his in Nicholson's Journal, endeavoured to sup- 

 port the opinion of atmospheric air being a chemical compound; 

 but he does not avail b.imself of the two affinities of the strong 

 and the weak, in order to explain the phtenomena of mixed gases." 

 Here ends Mr. Dalton's old detail; and had he given a true his- 

 tory one might probably read it without disgust. 



The distinction of weak and strong affuiity is of considerable 

 importance in Nature, and they should be well understood and 

 defined from each other. What is called vveak affinity does not 

 oidy prevail between j)ermanent gases, but also between those 

 jrases and the vapour of water and a!! other vapours, and even 

 between llnids and solid substances. 



I will now prove that 1 was the first who made this important 

 distinction between weak and strong affinity twenty-eight years 

 ago. At that distant period I felt doubtful whether the suljduir 

 and hydrogen in sulphuretted hydrogen gas were chemically 

 united or not, from the great facility with which they were se- 

 parated in a variety of ways. This consideration led to the fol- 

 lowing remarks, which I (juote from page 73 of my Comparative 

 Vhiu. 



" In my opinion it is mere solution (that is, that the sulphur 

 is hell in .solution by the hydrogen), such as takes place between 

 the neutral salts and water, the alkalies and water, and sugar and 

 water, &c. Although the facility with which sulphuretted iiy- 

 drogeu is decomposed favours this hypothesis, yet there are cir- 

 cumstances api)arently against it, |)articuhirly its condensaticm 

 in water, and its expulsion from it again by heat without decom- 

 position. 



" L'|)on what principle this modification of attraction exists 

 Q 2 between 



