On the mechanical ConAenj ation and RarefacTton of Air. 59 



and filver in contact with each other, but without adhe- 

 fion*. 



It was this purified cobalt which I employed to difcover 

 whether it was fufceptible of acquiring polarity by the mag- 

 net ; and to afcertain it fully, I begged our colleague Cou- 

 lomb, who is fo familiar with thefe experiments, to have 

 the goodnefs to aflilt me. I carried to him the ingots of 

 purified cobalt and nickel, which he endeavoured to mag- 

 netize; but, as they did not indicate any magnetic property 

 in a fatisfa&ory manner, he defired me to fufc them into 

 thin plates, which I accordingly did. They were then eafily 

 magnetized, and, being fufpended merely by a filk thread, 

 indicated the poles in a perfect manner. Thefe fa&s prove 

 that thefe two femi-metallic fubftances have, as well as iron, 

 the faculty of retaining the magnetic fluid introduced into 

 their pores. 



XIII. Experiments and Obfervations on the Heat and Cold 

 produced by the mechanical Condenfation and Rarefaclion 

 of Air, By John Dalton f. 



IF a thermometer be inclofed in a receiver and the air 

 fuddenly condenfed, the thermometer rifes a few degrees 

 above the temperature of the atmofphere; and if the air be 

 exhaufted from a receiver inclofing a thermometer, the mer- 

 cury finks a few degrees immediately ; but in both cafes after 

 fome time it refumes its former ftation. Thefe facls are well 

 known to philofophers of the prefent age, but they do not all 

 agree in the explanation of them. Thinking the fubje6t 

 worthy of elucidation, I was induced to inftitute a feries of 

 experiments for the purpofe, which I apprehend have led to a 

 clear demonftration of the caufe of the phasnomena, and more- 

 over make the facts th em felves appear in a fomewhat different 

 point of view from what they are feen in at the rirfl moment. 

 One circumiiance is very remarkable, that whether the 

 mercury rifes or falls in thefe inflances, it is done very ra- 

 pidly; whereas in the open air, if a thermometer be only- 

 two or three degrees above or below the temperature, it 

 moves very flowly. This feems to have fuggeited to every 

 one the idea that the elafticity of the glafs bulb of the ther- 

 mometer has a principal fhare in producing the effecl:, by 



* I purified the nickel from the iron it contained by fublimation with 

 ( il-ammoniac, and reduced it by fufing it with black flux, 

 -t From the Mu'icb'jler Memoir s, vol. v. part i. 



caufing 



