280 PHILOSOPHICAL TRANSACTIONS. [ANNO 1782. 



sive increase of heat, and piled in their order on each other in a glass tube, ex- 

 hibited a regular and pretty extensive series of colours ; from a flesh-colour to a 

 deep brownish red, thence to a chocolate, and so on to nearly black, with all 

 the intermediate tints between these colours. A back being fixed to the tube, 

 like the scale of a thermometer, and the numbers of the pieces marked on it 

 respectively opposite to them, it is obvious, that these numbers may be con- 

 sidered as so many thermometric divisions or degrees ; and that, if another piece 

 of the same composition be fired in any other kiln or furnace, not exceeding the 

 utmost heat of the first, it will acquire a colour corresponding to some of the 

 pieces in the tube, and thus point out the degree of heat which that piece, and 

 consequently such other matters as were in the fire along with it, have 

 undergone. 



It must however be confessed, that for general use a thermometer on this 

 principle is liable to objection, as ideas of colours are not perfectly communicable 

 by words ; nor are all eyes, or all lights, equally adapted for distinguishing them, 

 especially the shades which approach near to each other ; and the effects of phlo- 

 gistic vapours, in altering the colour, may not in all cases be easily guarded 

 against. 



In considering this subject attentively, another property of argillaceous bodies 

 occurred to me ; a property which obtains, in a greater or less degree, in every 

 kind of them that has come under my examination, so that it may be deemed a 

 distinguishing character of this order of earths : I mean, the diminution of 

 their bulk by fire ; I have the satisfaction to find, in a course of experiments 

 lately made with this view, that it is a more accurate and extensive measure of 

 heat than the different shades of colour. I have found, that this diminution 

 begins to take place in a low red heat ; and that it proceeds regularly, as the 

 heat increases, till the clay becomes vitrified, and consequently to the utmost 

 degree that crucibles, or other vessels made of this material, can support. The 

 total contraction of some good clays, which I have examined, in the strongest 

 of my own fires, is considerably more than i part in every dimension. 



If, therefore, we can procure at all times a clay sufficiently apyrous or un- 

 vitrescible, and always of the same quality in regard to contraction by heat ; and 

 if we can find means of measuring this contraction with ease and minute accu- 

 racy, I flatter myself, that we shall be furnished with a measure of fire sufficient 

 for every purpose of experiment or business. We have, in different parts of 

 England, immense beds of clay ; each of which, at equal depths, is pretty uni- 

 form in quality throughout its whole extent. Of all the sorts I have hitherto 

 tried, some of the purest Cornish porcelain clays seem the best adapted, both 

 for supporting the intensity, and measuring the degrees, of fire. For preparing 

 and applying this material to thennoinetric purposes, the following method is 



