8 HISrORr of the SOCIETT. 



of the objed of the DifTertation, and of fome of the reafonings 

 employed in it. 



Dr Hutton was led into the fpeculations contained in the 

 Differtation, by an account of two experiments made by M M. de 

 Saussure and Pictet of Geneva. In the firft of thefe experi- 

 ments, two concave fpecula were placed oppofite and parallel to 

 one another, about twelve feet diftant ; and in the focus of one of 

 them was a ball of iron, which had been heated to incandefcence, 

 but allowed to cool till it was no longer luminous, even in the 

 dark. In the focus of the other fpeculum a thermometer was 

 placed, which prefently rofe 8° (of Reaumur's fcale) above 

 another that flood near it, but without the focus. Voyages dans 

 les jilpes, torn. II. § 926. 



To account for this phenomenon, M. de Saussure fuppofes, 

 that there exifts what M. Lambert and fome other philofophers 

 have called radiant heat, and that this heat may be obfcure, and 

 not accompanied with light. This radiant heat he conceives to 

 be refle(5led in the fame manner that light is, and by that means 

 to have produced the effecft on the thermometer that has juft 

 been defcribed. 



To this folution Dr Hutton objedls, alleging, that it afcribes 

 properties or capacities to heat which are inconfiflent altogether 

 with our notion^ of it. "We know heat only as a quality of 

 bodies, and as adling either in expanding them, when it is call- 

 ed fenfible heat, or in giving them fluidity, when it is termed la- 

 tent heat. We never perceive it as exifling in any other Ihape, 

 and therefore, to fuppofe it capable of moving through fpace, 

 independently of body, and of being reflecfted from a polifhed 

 furface, is to afcribe to heat properties not predicable of it, and 

 quite inconfiftent with its nature, fo far as we have information 

 concerning it. 



Dr Hutton therefore propofes another explanation. From 

 experiments which he had made, long fince, he had found that 



the 



