LIFE of Dr BUTTON. 6j 



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After the period of the two publications juft mentioned, Dr 



Hutton made feveral excurfions into different parts of Scotland, 

 with a view of comparing certain remits of his theory of the 

 earth with actual obfervation. His account of granite, viz. that it 

 is a fubftance which, having been reduced into fufion by fub ter- 

 raneous heat, has been forcibly injected among the flrata alrea- 

 dy confolidated, w r as fo different from that of other mineralo- 

 gifts, that it feemed particularly to require farther examination. 

 He concluded, that if this account was juft, fome confirmation 

 of it muft appear at thofe places where the granite and the flrata 

 are in contact, or where the former emerges from beneath the 

 latter. In fuch fituations, one might expect veins of the flone 

 which had been in fufion to penetrate into the flone which 

 had been folid ; and fome imperfect defcriptions of granitic 

 veins gave reafon to imagine that this phenomenon was actu- 

 ally to be obferved. Dr Hutton was anxious that an injlantia 

 crucis might fubject his theory to the fevereft tefl. 



One 



latent, it will become colder than before. Thus alfo, when a quantity of heat 

 afcends by any means whatever, from one ftratum of air to a fuperior ftratum, a 

 part of it becomes latent, fo that an equilibrium of heat can never be eftablifhed 

 among the flrata ; but thofe which are lefs, mult always remain colder than thofe 

 that are more, comprefied. This was Dr Button's explanation, and it contains 

 no hypothetical principle whatfoever. 



To one who confiders meteorology with attention, the want of an accurate hygro- 

 meter can never fail to be a fubject of regret. The way of fupplying this deficien- 

 cy which Dr Hutton pra£tifed was by moiflening the ball of a thermometer, and 

 obferving the degree of cold produced by the evaporation of the moifture. The 

 degree of cold, cceteris paribus, will be proportional to the drynefs of the air, and 

 affords, of courfe, a meafure of that drynefs. The fame contrivance, but without 

 any communication whatfoever, occurred afterwards to Mr Leslte, and beino- 

 purfued through a feries of very accurate and curious experiments, has produced 

 an inftrument which promifes to anfwer all the purpofes of photometry, as well 

 as hygrometry, and fo to make a very important addition to our phyfical appara- 

 tus. 



